igb: Clear EICR bits for delayed MSI-X interrupts

Section 7.3.4.1 says:
> When auto-clear is enabled for an interrupt cause, the EICR bit is
> set when a cause event mapped to this vector occurs. When the EITR
> Counter reaches zero, the MSI-X message is sent on PCIe. Then the
> EICR bit is cleared and enabled to be set by a new cause event

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>