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: If4d40f7f56368aaa6f76149227ed72413f08a686
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>