net: dsa: mv88e6xxx: Fix mv88e6393x EPC write command offset

According to datasheet, the command opcode must be specified
into bits [14:12] of the Extended Port Control register (EPC).

Fixes: de776d0d ("net: dsa: mv88e6xxx: add support for mv88e6393x family")
Signed-off-by: Marco Migliore <m.migliore@tiesse.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/port.h

@@ -276,7 +276,7 @@
 /* Offset 0x10: Extended Port Control Command */
 #define MV88E6393X_PORT_EPC_CMD		0x10
 #define MV88E6393X_PORT_EPC_CMD_BUSY	0x8000
-#define MV88E6393X_PORT_EPC_CMD_WRITE	0x0300
+#define MV88E6393X_PORT_EPC_CMD_WRITE	0x3000
 #define MV88E6393X_PORT_EPC_INDEX_PORT_ETYPE	0x02
 
 /* Offset 0x11: Extended Port Control Data */