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

[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 <[email protected]>
Signed-off-by: Hans Verkuil <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>