iio: imu: adis: protect initial startup routine with state lock

The initial startup routine is called by some ADIS drivers during probe,
and before registering with IIO. Normally, userspace should not be able to
do any access to the device (as there shouldn't be any available).

This change extends the state lock to the entire initial-startup routine.
Behaviourally nothing should change, but this should make the library
function a bit more robust.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>