iio: imu: adis16400: rework locks using ADIS library's state lock

iio: imu: adis16400: rework locks using ADIS library's state lock

This change removes the use of indio_dev's mlock in favor using the state
lock from the ADIS library.

The set_freq() & get_freq() hooks are unlocked, so they require specific
locking. That is because in some cases the get_freq() hook is used in
combination with adis16400_set_filter().

In cases where only one read/write is done, the functions that hold the
state lock are used.

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