V4L/DVB (6407): planb: fix obvious interrupt handling bugs

irq handlers have returned a return value for years now... catch up with
the times.

Also, ditch unneeded prototype.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>