On old powermacs, it's possible that we get a stale edge interrupt when
doing request_irq(), that typically happens with the DBDMA controller
interrupts when the device was used by the firmware for booting.

I just tracked down a nasty memory corruption problem where that was
causing the bmac driver to try to process packets before the driver
internal data structures were properly initialized.

While I agree that the driver should (and will) be made more robust to
such things, Paulus and I decided that it makes little sense to keep
track of an "old" edge interrupt that happens before a driver does
request_irq.  (On those powermacs, those are only the DBDMA interrupts
anyway, and none of the DBDMA users will care). 

This patch implements a "startup" handler for the old Apple PIC that
will "ack" pending edge interrupts before unmasking, thus preventing
those stale interrupts to be delivered.

It also "fixes" the ppc32 irq core to call the startup() callback when
available instead of just calling enable().
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>