V4L/DVB (13188): xc5000: return an error on tuning attempts if firmware not loaded

Xc5000 tuning attempts shouldn't return zero in the case where the firmware
did not load successfully.

Thanks to Michael Krufky for pointing out this issue.

Cc: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>