ASoC: refactor snd_soc_update_bits()

Introduce a wrapper call snd_soc_update_bits_locked()
that will take the codec mutex. This call is used
when the codec mutex is not already taken.

Drivers calling snd_soc_update_bits() may wish to
make sure the codec mutex is taken from the driver.

Signed-off-by: Eero Nurkkala <ext-eero.nurkkala@nokia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>