mlxsw: spectrum_fid: Split a helper out of mlxsw_sp_fid_flood_table_mid()

In future patches, for CFF flood mode support, we will need a way to
determine a PGT base dynamically, as an op. Therefore, for symmetry,
split out a helper, mlxsw_sp_fid_pgt_base_ctl(), that determines a PGT base
in the controlled mode as well.

Now that the helper is available, use it in mlxsw_sp_fid_flood_table_init()
which currently invokes the FID->MID helper to that end.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/fd41c66a1df4df6499d3da34f40e7b9efa15bc3e.1701183892.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>