staging:iio:ad9834: Mark transfer buffers as __b16

Fixes the following warnings from sparse:
	drivers/staging/iio/frequency/ad9834.c:45:26: warning: incorrect type in assignment (different base types)
	drivers/staging/iio/frequency/ad9834.c:45:26:    expected unsigned short [unsigned] [short] <noident>
	drivers/staging/iio/frequency/ad9834.c:45:26:    got restricted __be16 [usertype] <noident>
	drivers/staging/iio/frequency/ad9834.c:47:26: warning: incorrect type in assignment (different base types)
	drivers/staging/iio/frequency/ad9834.c:47:26:    expected unsigned short [unsigned] [short] <noident>
	drivers/staging/iio/frequency/ad9834.c:47:26:    got restricted __be16 [usertype] <noident>
	drivers/staging/iio/frequency/ad9834.c:59:18: warning: incorrect type in assignment (different base types)
	drivers/staging/iio/frequency/ad9834.c:59:18:    expected unsigned short [unsigned] data
	drivers/staging/iio/frequency/ad9834.c:59:18:    got restricted __be16 [usertype] <noident>
	drivers/staging/iio/frequency/ad9834.c:100:26: warning: incorrect type in assignment (different base types)
	drivers/staging/iio/frequency/ad9834.c:100:26:    expected unsigned short [unsigned] data
	drivers/staging/iio/frequency/ad9834.c:100:26:    got restricted __be16 [usertype] <noident>
	drivers/staging/iio/frequency/ad9834.c:108:26: warning: incorrect type in assignment (different base types)
	drivers/staging/iio/frequency/ad9834.c:108:26:    expected unsigned short [unsigned] data
	drivers/staging/iio/frequency/ad9834.c:108:26:    got restricted __be16 [usertype] <noident>
	drivers/staging/iio/frequency/ad9834.c:122:26: warning: incorrect type in assignment (different base types)
	drivers/staging/iio/frequency/ad9834.c:122:26:    expected unsigned short [unsigned] data
	drivers/staging/iio/frequency/ad9834.c:122:26:    got restricted __be16 [usertype] <noident>
	drivers/staging/iio/frequency/ad9834.c:131:26: warning: incorrect type in assignment (different base types)
	drivers/staging/iio/frequency/ad9834.c:131:26:    expected unsigned short [unsigned] data
	drivers/staging/iio/frequency/ad9834.c:131:26:    got restricted __be16 [usertype] <noident>
	drivers/staging/iio/frequency/ad9834.c:194:26: warning: incorrect type in assignment (different base types)
	drivers/staging/iio/frequency/ad9834.c:194:26:    expected unsigned short [unsigned] data
	drivers/staging/iio/frequency/ad9834.c:194:26:    got restricted __be16 [usertype] <noident>
	drivers/staging/iio/frequency/ad9834.c:387:18: warning: incorrect type in assignment (different base types)
	drivers/staging/iio/frequency/ad9834.c:387:18:    expected unsigned short [unsigned] data
	drivers/staging/iio/frequency/ad9834.c:387:18:    got restricted __be16 [usertype] <noident>

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>