net: dsa: mv88e6xxx: get rid of SPEED_MAX setting

Currently, all the device specific speed setting functions convert
SPEED_MAX to the actual speed of the port. Rather than having each
of the mv88e6xxx chip specifics handling SPEED_MAX, derive it from
the mac_capabilities instead.

This is only needed for CPU and DSA ports, so move the logic up into
mv88e6xxx_setup_port() - which allows us to kill off all users of
SPEED_MAX throughout the driver.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>