Output volume settings from mixer controls would be lost.

Imagine that "Headphone Volume" has set to -10dB via amixer:
- in mtk_hp_enable()
  - hp_store_gain() saves the volume setting -10dB from regmap_read()
    to ana_gain[AUDIO_ANALOG_VOLUME_HPOUTL]
  - headset_volume_ramp() ramps up from -10dB to -10dB
- in mtk_hp_disable()
  - headset_volume_ramp() ramps down from -10dB to -40dB

Next time in mtk_hp_enable(), hp_store_gain() would save -40dB but
not -10dB.  As a result, headset_volume_ramp() would ramp from -10dB to
-40dB (which is mute).
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Mark Brown <broonie@kernel.org>