mISDN: fix the races with timers going off just as they are deleted

timer callback in timerdev.c both accesses struct mISDNtimer it's
called for *and* moves it to dev->expired.  We need del_timer_sync(),
or we risk kfree() freeing it right under dev_expire_timer() *and*
dev->expired getting corrupted.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>