ixgbe: Fix SR-IOV MTU warning

This change corrects the fact that we were using 1522 to test for the
max frame size in ixgbe_change_mtu and 1518 in ixgbe_set_vf_lpe.  The
difference was the addition of VLAN_HLEN which we only need to add in the case
of computing a buffer size, but not a filter size.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Sibai Li <Sibai.li@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>