iio: amplifiers: ad8366: use own lock to guard state

This driver is still using iio_dev's mlock to guard against inconsistent
state. This has been discouraged for some time.

This change switches to using it's own mutex, defined on the state struct.

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