ASoC: rt5645: Lock mutex in rt5645_enable_push_button_irq

rt5645_enable_push_button_irq uses snd_soc_dapm_*_unlocked
functions, so it needs to lock the required dapm mutex.

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>