[media] bttv: remove custom_irq and gpioq from bttv struct

The RC5 old decoder used custom_irq to indicate the need of handling
the IRQ on a different way. Instead of doing it, let the core just call the
bttv input IRQ handler, and add the code there to call the legacy decoder.

While here, remove the gpioq waitqueue, as this is not used anywhere, and
add a debug msg to help removing the legacy RC5 code.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>