V4L/DVB (7325): cx88-dvb: fix an OOPS for xc3028 devices, when dvb_attach fails

If dvb_attach fails, dev->dvb.frontend is NULL. This will produce an OOPS, as
reported.

Thanks to Vanessa Ezekowitz <vanessaezekowitz@gmail.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>