drivers: iio: magnetometer: Fix sparse endianness warnings cast to restricted __be16

Fix the following sparse endianness warnings:

drivers/iio/magnetometer/ak8975.c:716:16: warning: cast to restricted __le16
drivers/iio/magnetometer/ak8975.c:837:19: warning: cast to restricted __le16
drivers/iio/magnetometer/ak8975.c:838:19: warning: cast to restricted __le16
drivers/iio/magnetometer/ak8975.c:839:19: warning: cast to restricted __le16

Signed-off-by: Sandhya Bankar <bankarsandhya512@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>