Merge branch 'dsa-mtu'

Andrew Lunn says:

====================
Adjust MTU of DSA master interface

DSA makes use of additional headers to direct a frame in/out of a
specific port of the switch. When the slave interfaces uses an MTU of
1500, the master interface can be asked to handle frames with an MTU
of 1504, or 1508 bytes. Some Ethernet interfaces won't
transmit/receive frames which are bigger than their MTU.

Automate the increasing of the MTU on the master interface, by adding
to each tagging driver how much overhead they need, and then calling
dev_set_mtu() of the master interface to increase its MTU as needed.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>