configs: add initial value for "ADC2 Volume"

Initial value for this mixer ctl is missed, this is
to fix an issue that, when VA session on headset mic
is disabled, it can be reset to 0 and causes low
volume for audio recording.
Also, valid volume range is from 0 to 20, assign
a valid value for it.

Change-Id: Ia6b71a09b3b7f4b7918caed5499e606e861a44d7