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>