media: igorplugusb: prevent use after free in probe error

The timer uses the URB. Free it only after the timer
has been stopped.

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>