net: mvneta: fix mtu change on port without link

It is incorrect to enable TX/RX queues (call by mvneta_port_up()) for
port without link. Indeed MTU change for interface without link causes TX
queues to stuck.

Fixes: c5aff182 ("net: mvneta: driver for Marvell Armada 370/XP
network unit")
Signed-off-by: Yelena Krivosheev <yelena@marvell.com>
[gregory.clement: adding Fixes tags and rewording commit log]
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>