iio: gyro: adis16136: rework locks using ADIS library's state lock

This replaces indio_dev's mlock with the state lock/mutex from the ADIS
library.

The __adis16136_get_freq() function has been prefixed to mark it as
unlocked. The adis16136_{set,get}_filter() functions now hold the state
lock for all the ops that they do.

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