ASoC: tpa6130a2: Use one event handler for PGA_E

Reduce the amount of duplicated code by using single
event handler for PGA_E to enable the needed channel.
Use the w->shift to pass the channel information to
the handler function.

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>