net: dsa: mv88e6xxx: rename new FID helper

Rename the _mv88e6xxx_fid_new helper to mv88e6xxx_atu_new to get rid of
the old underscore prefix naming convention and be consistent with the
rest of the chip-wide ATU API.

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