mISDN: fix misdn_add_timer()/misdn_del_timer() race

do add_timer() *before* unlocking dev->lock, or unpleasant things can
happen if misdn_del_timer() on another CPU finds the sucker, calls
del_timer_sync() (which does nothing, since we hadn't started the
timer yet) and frees it, just as we get around to add_timer()...

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