dpaa2-eth: Use stored link settings

Whenever a link state change occurs, we get notified and save
the new link settings in the device's private data. In ethtool
get_link_ksettings, use the stored state instead of interrogating
the firmware each time.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>