Only switch families with 4096 address databases have dedicated FID
registers for ATU and VTU operations.

Factorize the access to the GLOBAL_ATU_FID register and introduce a
mv88e6xxx_has_fid_reg() helper function to protect the access to
GLOBAL_ATU_FID and GLOBAL_VTU_FID.
Signed-off-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>