iio: inv-mpu: Specify the expected format/precision for write channels

The gyroscope needs IIO_VAL_INT_PLUS_NANO for the scale channel and
unless specified write returns MICRO by default.
This needs to be properly specified so that write operations into scale
have the expected behaviour.

Signed-off-by: Adriana Reus <adriana.reus@intel.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>