net: mvpp2: do not set GMAC autoneg when using XLG MAC

When using the XLG MAC, it does not make sense to force the GMAC autoneg
parameters. This patch adds checks to only set the GMAC autoneg
parameters when needed (i.e. when not using the XLG MAC).

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