Input: fix locking in memoryless force-feedback devices

Now that input core acquires dev->event_lock spinlock and disables
interrupts when propagating input events, using spin_lock_bh() in
ff-memless driver is not allowed. Actually, the timer_lock itself
is not needed anymore, we should simply use dev->event_lock
as well.

Also do a small cleanup in force-feedback core.

Reported-by: kerneloops.org
Reported-by: http://www.kerneloops.org/searchweek.php?search=ml_ff_set_gain
Reported-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>