staging:iio:ad7746: Mark transfer buffer as __be32

Fixes the following warnings from sparse:
	drivers/staging/iio/cdc/ad7746.c:599:25: warning: cast to restricted __be32
	drivers/staging/iio/cdc/ad7746.c:599:25: warning: cast to restricted __be32
	drivers/staging/iio/cdc/ad7746.c:599:25: warning: cast to restricted __be32
	drivers/staging/iio/cdc/ad7746.c:599:25: warning: cast to restricted __be32
	drivers/staging/iio/cdc/ad7746.c:599:25: warning: cast to restricted __be32
	drivers/staging/iio/cdc/ad7746.c:599:25: warning: cast to restricted __be32

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