net: dsa: mv88e6xxx: use mv88e6185_g1_vtu_getnext() for the 6250

mv88e6250_g1_vtu_getnext is almost identical to
mv88e6185_g1_vtu_getnext, except for the 6250 only having 64 databases
instead of 256. We can reduce code duplication by simply masking off
the extra two garbage bits when assembling the fid from VTU op [3:0]
and [11:8].

Reviewed-by: Tobias Waldekranz <tobias@waldekranz.com>
Tested-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>