iio: imu: adis: group single conversion under a single state lock

The single conversion function does a series of reads + writes. This change
extends the use of the state_lock for the entire set of operations.
Previously, indio_dev's mlock was used. This change also removes the use of
this lock.

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