net: dsa: Fix scope of eeprom-length property

eeprom-length is a switch property, not a dsa property, and thus
needs to be attached to the switch node, not to the dsa node.

Reported-by: Andrew Lunn <andrew@lunn.ch>
Fixes: 6793abb4e849 ("net: dsa: Add support for switch EEPROM access")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>