e1000: FIX: be ready for incoming irq at pci_request_irq

DEBUG_SHIRQ code exposed that e1000 was not ready for incoming interrupts
after having called pci_request_irq. This obviously requires us to finish
our software setup which assigns the irq handler before we request the
irq.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>