media: imon: fix timer racing disconnect

The timer will report events for an input device.
Reporting events for an unregistered device is bad.
Hence the timer must be killed first.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>