net: mvpp2: only update the XLG configuration when needed

This patch improves the XLG configuration function, to only update the
XLG configuration register when a change is needed. This helps not
writing over and over the same XLG configuration each time phylink
request the MAC to be configured. This mimics the GMAC configuration
function.

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