ASoC: tlv320dac33: Control for line output gain

New control to select the line output gain.
This gain control affects the linein-to-lineout and
dac-to-loneout gain differently.
Use enum type to select the desired gain combination.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>