Input: exc3000 - properly stop timer on shutdown

We need to stop the timer on driver unbind or probe failures, otherwise
we get UAF/Oops.

Fixes: 7e577a17

 ("Input: add I2C attached EETI EXC3000 multi touch driver")
Reported-by: "Stahl, Michael" <mstahl@moba.de>
Link: https://lore.kernel.org/r/Y9dK57BFqtlf8NmN@google.com


Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>