igb: reduce size required to trigger low latency

Update the Adaptive Interrupt Moderation algorithm so that the low latency
state is triggered less easily to prevent high interrupt loads.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>