USB HID: hiddev - fix race between hiddev_send_event() and hiddev_release()

There is a small race window in which hiddev_release() could corrupt the
list that is being processed for new event in hiddev_send_event().
Synchronize the operations over this list.
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>