rtc: destroy mutex when releasing the device

Not destroying mutexes doesn't lead to resource leak but it's the correct
thing to do for mutex debugging accounting.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201110094205.8972-1-brgl@bgdev.pl