Input: ff-memless - kill timer in destroy()

No timer must be left running when the device goes away.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Reported-and-tested-by: syzbot+b6c55daa701fc389e286@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/1573726121.17351.3.camel@suse.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
