ASoC: TWL4030: Add helper function for output gain controls

Some of the gain controls in TWL (mostly those which are associated with
the outputs) are implemented in an interesting way:
 0x0 : Power down (mute)
 0x1 : 6dB
 0x2 : 0 dB
 0x3 : -6 dB
Inverting not going to help with these.
Custom volsw and volsw_2r get/put functions to handle these gains.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>