net: mvneta: fix error messages in mvneta_port_down function

This commit corrects error printing when shutting down the port.

[gregory.clement@free-electrons.com: split initial commit in two
individual changes]
Signed-off-by: Dmitri Epshtein <dima@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>