mlxsw: core, pci: Add plumbing related to CFF mode

CFF mode, for Compressed FID Flooding, is a way of organizing flood vectors
in the PGT table. The bus module determines whether CFF is supported, can
configure flood mode to CFF if it is, and knows what flood mode has been
configured. Therefore add a bus callback to determine the configured flood
mode. Also add to core an API to query it.

Since after this patch, we rely on mlxsw_pci->flood_mode being set, it
becomes a coding error if a driver invokes this function with a set of
fields that misses the initialization. Warn and bail out in that case.

The CFF mode is not used as of this patch. The code to actually use it will
be added later.

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/889d58759dd40f5037f2206b9fc4a78a9240da80.1700503644.git.petrm@nvidia.com


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