iio: imu: adis: add a note better explaining state_lock

The 'state_lock' mutex was renamed from 'txrx_lock' in a previous patch and
is intended to be used by ADIS drivers to protect the state of devices
during consecutive R/W ops.
The initial patch that introduced this change did not do a good [well, any]
job at explaining this. This patch adds a comment to the 'state_lock'
better explaining it's use.

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