V4L/DVB (7449): cx88: fix oops on module removal caused by IR worker

If the IR worker is not stopped before the removal of the cx88xx module,
an OOPS may occur, because the worker function cx88_ir_work gets called.
So stop the ir worker.

Signed-off-by: Marton Balint <cus@fazekas.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>