net: mvpp2: force the XLG MAC link up or down when not using in-band

This patch force the XLG MAC link state in the phylink link_up() and
link_down() helpers when not using in-band auto-negotiation. This mimics
what's already done for the GMAC and follows what's advised in the
phylink documentation.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>