Via southbridges use register 0x3c of the on-board devices (USB and
AC97) to control interrupt routing for those.  In drivers/pci/quirks.c we
set it correctly (dev->irq & 15).  However, in pirq_enable_irq() where the
second half of that stuff lives, we forget to apply the mask.

	That's what causes problems with ioapic on via motherboards in 2.6.
One-liner below ACKed by Alan, verified on via-based boxen here, obviously
doesn't affect non-via ones (we only set interrupt_line_quirk for via
chipsets).