[media] mantis_dvb: fix some error codes in mantis_dvb_init()

We should be returning negative error codes here or it leads to a crash.
This also silences a static checker warning.

	drivers/media/pci/mantis/mantis_cards.c:250 mantis_pci_probe()
	warn: 'mantis->dmxdev.dvbdev->fops' double freed

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>