Fix HTC headset handling.

HTC's headset driver adds additional bits to the switch state, which
we're unable to deal with. Fix that by masking out those bits and
keeping only the valid ones.

Based off: https://gerrit.omnirom.org/#/c/5500/

Original author retained :)

Change-Id: I1abca665488e89f76eae97c4aab86bd51afd2a19
Merge "Fix HTC headset handling." into kitkat