iio:ad5791: Mark transfer buffers as __be32

Fixes the following warnings from sparse:
	drivers/iio/dac/ad5791.c:114:18: warning: incorrect type in assignment (different base types)
	drivers/iio/dac/ad5791.c:114:18:    expected unsigned int [unsigned] [usertype] d32
	drivers/iio/dac/ad5791.c:114:18:    got restricted __be32 [usertype] <noident>
	drivers/iio/dac/ad5791.c:142:21: warning: incorrect type in assignment (different base types)
	drivers/iio/dac/ad5791.c:142:21:    expected unsigned int [unsigned] [usertype] d32
	drivers/iio/dac/ad5791.c:142:21:    got restricted __be32 [usertype] <noident>
	drivers/iio/dac/ad5791.c:144:21: warning: incorrect type in assignment (different base types)
	drivers/iio/dac/ad5791.c:144:21:    expected unsigned int [unsigned] [usertype] d32
	drivers/iio/dac/ad5791.c:144:21:    got restricted __be32 [usertype] <noident>
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32

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