net: dsa: mv88e6xxx: implement vtu_getnext and vtu_loadpurge for mv88e6250

These are almost identical to the 6185 variants, but have fewer bits
for the FID.

Bit 10 of the VTU_OP register (offset 0x05) is the VidPolicy bit,
which one should probably preserve in mv88e6xxx_g1_vtu_op(), instead
of always writing a 0. However, on the 6352 family, that bit is
located at bit 12 in the VTU FID register (offset 0x02), and is always
unconditionally cleared by the mv88e6xxx_g1_vtu_fid_write()
function.

Since nothing in the existing driver seems to know or care about that
bit, it seems reasonable to not add the boilerplate to preserve it for
the 6250 (which would require adding a chip-specific vtu_op function,
or adding chip-quirks to the existing one).

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>