iio: imu: adis: rename txrx_lock -> state_lock

iio: imu: adis: rename txrx_lock -> state_lock

The lock can be extended a bit to protect other elements that are not
particular to just TX/RX. Another idea would have been to just add a new
`state_lock`, but that would mean 2 locks which would be redundant, and
probably cause more potential for dead-locks.

What will be done in the next patches, will be to add some unlocked
versions for read/write_reg functions.

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