[media] s2255: Do not free fw_data until timer handler has actually stopped using it

Function del_timer() does not guarantee that timer was really deleted.
If the timer handler is beeing executed at the moment, the function
does nothing. So, we have a race between del_timer() and kfree(), and
it's possible to use already freed memory in the handler.

Signed-off-by: Kirill Tkhai <tkhai@yandex.ru>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>