media: bt8xx: Fix err 'bt878_probe()'

This is odd to call 'pci_disable_device()' in an error path before a
coresponding successful 'pci_enable_device()'.

Return directly instead.

Fixes: 77e0be12 ("V4L/DVB (4176): Bug-fix: Fix memory overflow")

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>