Merge branch 'Fix-MTU-warnings-for-fec-mv886xxx-combo'

Andrew Lunn says:

====================
Fix MTU warnings for fec/mv886xxx combo

Since changing the MTU of dsa slave interfaces was implemented, the
fec/mv88e6xxx combo has been giving warnings:

[    2.275925] mv88e6085 0.2:00: nonfatal error -95 setting MTU on port 9
[    2.284306] eth1: mtu greater than device maximum
[    2.287759] fec 400d1000.ethernet eth1: error -22 setting MTU to include DSA overhead

This patchset adds support for changing the MTU on mv88e6xxx switches,
which do support jumbo frames. And it modifies the FEC driver to
support its true MTU range, which is larger than the default Ethernet
MTU.
====================

Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>