ASoC: twl6040: Simplify custom get_volsw callback

The custom get_volsw does not need to call any core get_volsw calls,
since we are returning the shadow values for the gains.
Return -EINVAL in the unlikely event, if the function has been called
for unhandled control. This way we can remove one check in the code.

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