V4L/DVB: cx23885: Move AV Core irq handling to a work handler

Interrupts from the AV Core are best handled by a workqueue handler
since many I2C transactions are required to service the AV Core
interrupt.  The AV_CORE PCI interrupt is disabled by the IRQ handler
and reenabled when the work handler is finished.

Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>