net: dsa: mv88e6xxx: Set the CMODE for mv88e6390 ports 9 & 10

Unlike most ports, ports 9 and 10 of the 6390X family have configurable
PHY modes. Set the mode as part of adjust_link().

Ordering is important, because the SERDES interfaces connected to
ports 9 and 10 can be split and assigned to other ports. The CMODE has
to be correctly set before the SERDES interface on another port can be
configured. Such configuration is likely to be performed in
port_enable() and port_disabled(), called on slave_open() and
slave_close().

The simple case is port 9 and 10 are used for 'CPU' or 'DSA'. In this
case, the CMODE is set via a phy-mode in dsa_cpu_dsa_setup(), which is
called early in the switch setup.

When ports 9 or 10 are used as user ports, and have a fixed-phy, when
the fixed fixed-phy is attached, dsa_slave_adjust_link() is called,
which results in the adjust_link function being called, setting the
cmode. The port_enable() will for other ports will be called much
later.

When ports 9 or 10 are used as user ports and have a real phy attached
which does not use all the available SERDES interface, e.g. a 1Gbps
SGMII, there is currently no mechanism in place to set the CMODE of
the port from software. It must be hoped the stripping resistors are
correct.

At the same time, add a function to get the cmode. This will be needed
when configuring the SERDES interfaces.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>