mlxsw: spectrum_router: Add a helper to get subport number from a RIF

mlxsw: spectrum_router: Add a helper to get subport number from a RIF

In the CFF flood mode, responsibility for management of the PGT entries for
rFIDs is moved from FW to the driver. All rFIDs are based off either a
front panel port, or a LAG port. The flood vectors for port-based rFIDs
enable just the port itself, the ones for LAG-based rFIDs enable all member
ports of the LAG in question.

Since all rFIDs based off the same port have the same flood vector, and
similarly for LAG-based rFIDs, the flood entries are shared. The PGT
address of the flood vector is therefore determined based on the port (or
LAG) number of the RIF connected with the rFID.

Add a helper to determine subport number given a RIF, to be used in these
calculations.

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/d7ab43cf5b021f785f363f236e4b6780d10eea93.1700503644.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>