net: dsa: mv88e6131: Determine and use number of switch ports

Determine and use number of switch ports from chip ID instead of always
using the maximum, and return error when an attempt is made to access a
non-existing port.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/dsa/mv88e6xxx.h b/drivers/net/dsa/mv88e6xxx.h
index aca4879..ef05844 100644
--- a/drivers/net/dsa/mv88e6xxx.h
+++ b/drivers/net/dsa/mv88e6xxx.h
@@ -109,6 +109,7 @@
 	struct mutex eeprom_mutex;
 
 	int		id; /* switch product id */
+	int		num_ports;	/* number of switch ports */
 
 	/* hw bridging */