net: mvpp2: convert to use mac_prepare()/mac_finish()

Convert mvpp2 to use the mac_prepare() and mac_finish() methods in
preparation to converting mvpp2 to split-PCS support.

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