V4L/DVB (13961): em28xx-dvb: fix memleak in dvb_fini()

this patch fixes a memory leak which occurs when an em28xx card with DVB
extension is unplugged or its DVB extension driver is unloaded. In
dvb_fini(), dev->dvb must be freed before being set to NULL, as is done
in dvb_init() in case of error.
Note that this bug is also present in the latest stable kernel release.

Signed-off-by: Francesco Lavra <francescolavra@interfree.it>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>