mlxsw: Prepare 'bridge_type' field for SFMR usage

Under the legacy bridge model, the field 'bridge_type' is used only
in SFGC register, to determine the type of flood table (FID/FID offset).

Under the unified bridge model, it will be used also in SFMR register.
When a BUM packet needs to be flooded, SFGC is used to provide the
'mid_base' for PGT table. The access to SFGC is by
{packet type, bridge type}. Under the unified bridge model, software is
responsible for configuring 'bridge_type' as part of SFMR.

As preparation for the new required configuration, rename
'enum mlxsw_reg_sfgc_bridge_type' to 'enum mlxsw_reg_bridge_type'. Then
it can be used also in SFMR. In addition, align the names of the values to
internal documentation.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>