e1000: fix NAPI performance on 4-port adapters

This fix attempts to solve a customer (IBM) reported issue with NAPI
enabled e1000 having bad performance when transmitting simultaneously
on four ports.  The issue comes down to an interaction between NAPI,
hardware interrupt balancing, and the driver rescheduling poll on
the same processor.  Try to fix by allowing the driver to re-enable
interrupts sooner instead of polling one more time, when there was
recently all the work completed in cleanup.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>