sound: add sound control to increase headphones, speaker and mic gain.

Change-Id: Ifb2d6e6cd6370908a9392c2ccc3bc3ad0e7d957b
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
sound: wcd9320: tidy up things a little bit.

Only accept signed values from now on.
The 'headset gain' will only be used to remove some gain noise,
increasing it beyond 1 step above the default doesn't make a
difference so rather just have it decrease the gain.

Change-Id: Ie773cb19ac518cad0887e30e6983b251b11b8769
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>