net: dsa: mv88e6xxx: support RMII cmode

Add support for direct RMII MAC mode. This allows hardware with CPU port
connected in direct 100M fixed link to work properly.

Signed-off-by: Baruch Siach <baruch.siach@siklu.com>
Link: https://lore.kernel.org/r/a962d1ccbeec42daa10dd8aff0e66e31f0faf1eb.1646050203.git.baruch@tkos.co.il
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/mv88e6xxx/port.c

@@ -550,6 +550,9 @@ static int mv88e6xxx_port_set_cmode(struct mv88e6xxx_chip *chip, int port,
 		mode = PHY_INTERFACE_MODE_1000BASEX;
 
 	switch (mode) {
+	case PHY_INTERFACE_MODE_RMII:
+		cmode = MV88E6XXX_PORT_STS_CMODE_RMII;
+		break;
 	case PHY_INTERFACE_MODE_1000BASEX:
 		cmode = MV88E6XXX_PORT_STS_CMODE_1000BASEX;
 		break;