dsa: mv88e6xxx: Drop duplicate declaration of 'ret' variable

A duplicate declaration of 'ret' can result in hiding an error code.
Drop it.

Fixes: 17ee3e04 ("net: dsa: Provide additional RMON statistics")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>