net: dsa: mv88e6xxx: make VTU helpers static

Now that we have chip operations for VTU accesses, mark all helpers from
global1_vtu.c as static. Only the various implementations of the
GetNext, LoadPurge and Flush operations need to be exposed.

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>