drivers: mpu9250: magnetometer data read fix

The magnetometer data read from AK9863 is assumed to be big endian
(like the accelerometer & gyroscope data).
However it is in little endian, which means the appropriate byteorder
function is sys_le16_to_cpu().
See the MPU925 register map:
https://3cfeqx1hf82y3xcoull08ihx-wpengine.netdna-ssl.com/wp-content/uploads/2017/11/RM-MPU-9250A-00-v1.6.pdf


At 5.6 HXL to HZH: Measurement Data

Signed-off-by: Loris Schmit <loris.schmit@inovex.de>