dsa: Rename mv88e6123_61_65 to mv88e6123 to be consistent

All the drivers support multiple chips, but mv88e6123_61_65 is the
only one that reflects this in its naming. Change it to be consistent
with the other drivers.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>