r8169: prevent RxFIFO induced loops in the irq handler.

While the RxFIFO interruption is masked for most 8168, nothing prevents
it to appear in the irq status word. This is no excuse to crash.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Ivan Vecera <ivecera@redhat.com>
Cc: Hayes <hayeswang@realtek.com>