iio: gyro: adis16260: replace mlock with ADIS lib's state_lock

This change uses the ADIS library's state_lock to protect the state of the
`max_speed_hz` change that is done during the set of the sampling
frequency.

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