staging:iio:adxrs450: Make transfer buffers __be32

Fixes the following sparse warnings:

	drivers/staging/iio/gyro/adxrs450_core.c:46:15: warning: cast to restricted __be32
	drivers/staging/iio/gyro/adxrs450_core.c:62:17: warning: cast to restricted __be32
	drivers/staging/iio/gyro/adxrs450_core.c:89:15: warning: cast to restricted __be32
	drivers/staging/iio/gyro/adxrs450_core.c:129:17: warning: cast to restricted __be32
	drivers/staging/iio/gyro/adxrs450_core.c:168:16: warning: cast to restricted __be32

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