igb: only support SRRCTL_DROP_EN when using multiple queues

The SRRCTL.DROP_EN bit should only be set when we are supporting
multiple queues. This bit is meant to prevent head of line blocking
and is unnecessary in the single queue case.

Signed-off-by: Nicholas Nunley <nicholasx.d.nunley@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>