iio:ad5504: Mark transfer buffers as __be16

Fixes the following warnings from sparse:
	drivers/iio/dac/ad5504.c:71:19: warning: incorrect type in initializer (different base types)
	drivers/iio/dac/ad5504.c:71:19:    expected unsigned short [unsigned] [usertype] tmp
	drivers/iio/dac/ad5504.c:71:19:    got restricted __be16 [usertype] <noident>
	drivers/iio/dac/ad5504.c:80:19: warning: incorrect type in initializer (different base types)
	drivers/iio/dac/ad5504.c:80:19:    expected unsigned short [unsigned] [usertype] tmp
	drivers/iio/dac/ad5504.c:80:19:    got restricted __be16 [usertype] <noident>
	drivers/iio/dac/ad5504.c:93:16: warning: cast to restricted __be16
	drivers/iio/dac/ad5504.c:93:16: warning: cast to restricted __be16
	drivers/iio/dac/ad5504.c:93:16: warning: cast to restricted __be16
	drivers/iio/dac/ad5504.c:93:16: warning: cast to restricted __be16

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