net: marvell: mvpp2: use mvpp2_is_xlg() helper elsewhere

There are several places which make the decision whether to access the
XLGMAC vs GMAC that only check for PHY_INTERFACE_MODE_10GKR and not its
XAUI variant.  Switch these to use the new helper so that we have
consistency through the driver.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>