[media] cx23885-dvb: Fix some issues at the DVB error handling

X-Patchwork-Delegate: m.chehab@samsung.com
As pointed by smatch:
	drivers/media/pci/cx23885/cx23885-dvb.c:1066 dvb_register() error: we previously assumed 'fe0->dvb.frontend' could be null (see line 1060)
	drivers/media/pci/cx23885/cx23885-dvb.c:1990 cx23885_dvb_register() error: we previously assumed 'fe0' could be null (see line 1975)

What happens is that the error handling logic when a frontend
register fails sometimes keep doing the work, as if it didn't
fail.

This could potentially cause an OOPS. So, simplify the logic
a little bit and return an error if frontend fails before
trying to setup VB2 queue.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>