ASoC: SOF: enable dual control for pga

Currently sof pga element supports only 1 kcontrol and you can't create
for example a mixer element with combined volume slider and mute switch.
So enable sof pga to have more than 1 kcontrol associated with it. Also
check for possible NULL tlv pointer as switch element might not have it.

Signed-off-by: Jaska Uimonen <jaska.uimonen@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191008164443.1358-8-pierre-louis.bossart@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>