net: mvneta: fix comment about phylink_speed_down

mvneta has switched to phylink, so the comment should look
like "We may have called phylink_speed_down before".

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Reviewed-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>