drivers: iio: dac: Fix sparse warning

Fixing below warnings:

drivers/iio/dac/ad5592r.c:58:41: warning: incorrect type in argument 2 (different base types)
drivers/iio/dac/ad5592r.c:58:41:    expected unsigned short [usertype] *buf
drivers/iio/dac/ad5592r.c:58:41:    got restricted __be16 *<noident>
drivers/iio/dac/ad5592r.c:62:41: warning: incorrect type in argument 2 (different base types)
drivers/iio/dac/ad5592r.c:62:41:    expected unsigned short [usertype] *buf
drivers/iio/dac/ad5592r.c:62:41:    got restricted __be16 *<noident>
drivers/iio/dac/ad5592r.c:92:41: warning: incorrect type in argument 2 (different base types)
drivers/iio/dac/ad5592r.c:92:41:    expected unsigned short [usertype] *buf
drivers/iio/dac/ad5592r.c:92:41:    got restricted __be16 *<noident>
drivers/iio/dac/ad5592r.c:110:41: warning: incorrect type in argument 2 (different base types)
drivers/iio/dac/ad5592r.c:110:41:    expected unsigned short [usertype] *buf
drivers/iio/dac/ad5592r.c:110:41:    got restricted __be16 *<noident>

Signed-off-by: Sandhya Bankar <bankarsandhya512@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>