[media] go7007: don't continue if firmware can't be loaded

The go7007 driver would continue during probe if no firmware could be
loaded. Without firmware the probe() should return an error, so do that.
Also move the registration of devices to the end of the probe() sequence:
once devices appear the full driver functionality should be available.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>