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: NDmitry Fleytman <dmitry@daynix.com>
Signed-off-by: NLeonid Bloch <leonid@daynix.com>
Tested-by: NAaron Brown <aaron.f.brown@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/e1000e/netdev.c

@@ -1959,8 +1959,10 @@ static irqreturn_t e1000_intr_msix_rx(int __always_unused irq, void *data)
 	 * previous interrupt.
 	 */
 	if (rx_ring->set_itr) {
-		writel(1000000000 / (rx_ring->itr_val * 256),
-		       rx_ring->itr_register);
+		u32 itr = rx_ring->itr_val ?
+			  1000000000 / (rx_ring->itr_val * 256) : 0;
+
+		writel(itr, rx_ring->itr_register);
 		rx_ring->set_itr = 0;
 	}