net: mvpp2: fix the phylink mode validation

The mvpp2_phylink_validate() sets all modes that are supported by a
given PPv2 port. An mistake made the 10000baseT_Full mode being
advertised in some cases when a port wasn't configured to perform at
10G. This patch fixes this.

Fixes: d97c9f4a ("net: mvpp2: 1000baseX support")
Reported-by: Russell King <linux@armlinux.org.uk>
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>