igb: Do not factor VLANs into RLPML calculation

The RLPML registers already take the size of VLAN headers into account when
determining the maximum packet length.  This is called out in EAS documents
for several parts including the 82576 and the i350.  As such we can drop
the addition of size to the value programmed into the RLPML registers.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>