net: marvell: mvpp2: use phy_interface_mode_is_8023z() helper

Use the phy_interface_mode_is_8023z() helper for detecting interface
modes that use 802.3z serial encoding.  This is equivalent to testing
for both 1000base-X and 2500base-X.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>