regmap: destroy mutex (if used) in regmap_exit()

While not destroying mutexes doesn't lead to memory leaks, it's still
the correct thing to do for mutex debugging accounting.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20200928120614.23172-1-brgl@bgdev.pl


Signed-off-by: Mark Brown <broonie@kernel.org>