net: dsa: add support for mac_prepare() and mac_finish() calls

Add DSA support for the phylink mac_prepare() and mac_finish() calls.
These were introduced as part of the PCS support to allow MACs to
perform preparatory steps prior to configuration, and finalisation
steps after the MAC and PCS has been configured.

Introducing phylink_pcs support to the mv88e6xxx DSA driver needs some
code moved out of its mac_config() stage into the mac_prepare() and
mac_finish() stages, and this commit facilitates such code in DSA
drivers.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>