net: dsa: Set the master device's MTU to account for DSA overheads

DSA tagging of frames sent over the master interface to the switch
increases the size of the frame. Such frames can then be bigger than
the normal MTU of the master interface, and it may drop them. Use the
overhead information from the tagger to set the MTU of the master
device to include this overhead.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>