e1000e: fix division by zero on jumbo MTUs

This patch fixes possible division by zero in receive
interrupt handler when working without adaptive interrupt
moderation.

The adaptive interrupt moderation mechanism is typically
disabled on jumbo MTUs.

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Leonid Bloch <leonid@daynix.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>