The absence of a correct offset leads an incorrect ODR mode
readback after use a hexadecimal number to mark the value from
FXOS8700_CTRL_REG1.

Get ODR mode by field mask and FIELD_GET clearly and conveniently.
And attach other additional fix for keeping the original code logic
and a good readability.

Fixes: 84e5ddd5 ("iio: imu: Add support for the FXOS8700 IMU")
Signed-off-by: Carlos Song <carlos.song@nxp.com>
Link: https://lore.kernel.org/r/20230118074227.1665098-2-carlos.song@nxp.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>