net: dsa: mv88e6xxx: move VTU FID accessors

Add helpers to access the VTU FID register in the global1_vtu.c file.

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>