Input: cm109 - fix handling of volume and mute buttons

The CM109 driver reported key press events of volume up / down and
record / playback mute buttons, but no release events. Report those events
properly by handling volume and mute keys seperately. For the record and
playback mute buttons, only presses are registered by the CM109, therefore
simulate press-n-release. This fixes the volume control buttons of various
USB headsets.

Signed-off-by: Florian Euchner <florian.euchner@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>