iio: imu: adis: add unlocked read/write function versions

This will allow more flexible control to group reads & writes into a single
lock (particularly the state_lock).

The end-goal is to remove the indio_dev->mlock usage, and the simplest fix
would have been to just add another lock, which would not be a good idea on
the long-run.

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