igb: Clear-on-read ICR when ICR.INTA is set

igb: Clear-on-read ICR when ICR.INTA is set

For GPIE.NSICR, Section 7.3.2.1.2 says:
> ICR bits are cleared on register read. If GPIE.NSICR = 0b, then the
> clear on read occurs only if no bit is set in the IMS or at least one
> bit is set in the IMS and there is a true interrupt as reflected in
> ICR.INTA.

e1000e does similar though it checks for CTRL_EXT.IAME, which does not
exist on igb.

Suggested-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>