ASoC: rt5640: Fix NULL dereference on module unload

The rt5640->jack is NULL if jack is already disabled at the time of
driver's module unloading.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20200106014707.11378-1-digetx@gmail.com


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