ixgbe: Set Drop_EN bit when multiple Rx queues are present w/o flow control

The drop enable bit can be used to improve the performance of the adapter
in the case of multiple queues being present.  This performance gain is due
to the fact that some slower CPUs can cause the FIFO to backfill preventing
faster CPUs from receiving additional work.  By setting the drop enable bit
we prevent this and instead just drop the packets that would have been
bound for the slower CPU.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>