i40e: do not flush after re-enabling interrupts

Hot path doesn't need read-flush after interrupt enable, and this
flush really causes a lot of extra cpu utilization.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>