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