[mvebu]: fix mvneta vlan tagging

[mvebu]: fix mvneta vlan tagging

This is a 3.14.x backport of:
"mvneta: Fix TSO and checksum for non-acceleration vlan traffic"

Upstream commits:

817dbfa5d1bc276a72c1a577310382008e8aca0a
2d39d120781a5770573dc6ed672a5a562f541aea

Fixes vlan tagging on SoC interfaces for the WRT1900AC.  Useful
when using mvsw6171 driver.

Signed-off-by: Mark Milburn <mark.a.milburn@gmail.com>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43781 3c298f89-4303-0410-b956-a3cf2f4a3e73