iio: core: add missing mutex_destroy in iio_dev_release()

iio: core: add missing mutex_destroy in iio_dev_release()

Add missing mutex_destroy() call in iio_dev_release() to properly
clean up the mutex initialized in iio_device_alloc(). Ensure proper
resource cleanup and follows kernel practices.

Found by code review.

While at it, create a lockdep key before mutex initialisation.
This will help with converting it to the better API in the future.

Fixes: 847ec80bbaa7 ("Staging: IIO: core support for device registration and management")
Fixes: ac917a81117c ("staging:iio:core set the iio_dev.info pointer to null on unregister under lock.")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>