ar71xx: ag71xx: increase calculated max frame length value

The r39147 commit introduces a regression: at lease on some routers
with ar8216 switch large packets get lost if 802.1q tagged port is
used on the interface connected to the aforementioned switch.

The r39147 changes code in the way so interface is set to accept
packets no longer than max ethernet frame length for a given mtu.

Unfortunately ar8216 has a feature: it sends two additional bytes
as a packet header and those this header needs to be added to the
max frame length. Otherwise long enough packets get lost.

The problem only manuifests itself if interface is used in vlan
tagged mode. If interface is untagged then ar8216's header fits
into space used by 802.1q tag and not packets are lost.

Include two additional bytes in the max frame length calculation
to fix the issue.

This patch is tested and works with Trendnet TEW-632BRP.

Signed-off-by Nikolay Martynov <mar.kolya@gmail.com>
Patchwork: http://patchwork.openwrt.org/patch/4656/
[juhosg:
  - simplify the patch to include the additional bytes of the
    switch header unconditionally,
  - change subject and update commit message]
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39219 3c298f89-4303-0410-b956-a3cf2f4a3e73