CHROMIUM: Input: cyapa - refactor debugfs mutex init/destruction

CHROMIUM: Input: cyapa - refactor debugfs mutex init/destruction

Make debugfs mutex init conditional on successful debugfs init, and
destroy mutex when debugfs is removed.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=chromium-os:23454
TEST=builds clean; cyapa debugfs entry read_fw still dumps firmware.

Change-Id: Iada91346af524521e2642d461aa0bc55753666a5
Reviewed-on: https://gerrit.chromium.org/gerrit/21454
Commit-Ready: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>