net: dsa: mv88e6xxx: implement port_set_speed for mv88e6250

The data sheet also mentions the possibility of selecting 200 Mbps for
the MII ports (ports 5 and 6) by setting the ForceSpd field to
0x2 (aka MV88E6065_PORT_MAC_CTL_SPEED_200). However, there's a note
that "actual speed is determined by bit 8 above", and flipping back a
page, one finds that bits 13:8 are reserved...

So without further information on what bit 8 means, let's stick to
supporting just 10 and 100 Mbps on all ports.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>