net: dsa: Restore MTU on master device on unload

A previous change tries to set the MTU on the master device to take
into account the DSA overheads. This patch tries to reset the master
device back to the default MTU.

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