media: cx231xx-dvb: fix memory leak in dvb_fini()

In dvb_init(), dev->dvb is allocated by kzalloc.
Therefore, it must be freed being set to NULL.

Signed-off-by: Young Xiao <92siuyang@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>