mlxsw: spectrum: Make vFID struct generic

Up until now we had a dedicated struct only for vFIDs, but before
introducing support for L3 interfaces we need to make it generic and
use it for all three types of FIDs:

1) FIDs - 0..4K-1, used for the VLAN-aware bridge
2) vFIDs - 4K..15K-1, used for VLAN-unaware bridges
3) rFIDs - 15K..16K-1, used to direct traffic to / from the router in
the device. Will be introduced later in the series.

The three types of L3 interfaces - Router InterFaces, RIFs - that will
be introduced correspond to the three types of FIDs and are configured
using them. Therefore, we'll need to store the links between them as
well as a reference count on the underlying FID, so that the
corresponding RIF will be destroyed when it reaches zero.

Note that the lower 0.5K vFIDs are currently used for for non-bridged
netdevs, so that traffic could be flooded to the CPU port. However, when
rFIDs will be introduced we'll no longer need these and they too will be
used for VLAN-unaware bridges.

Make the vFID struct generic by renaming it and some of its fields. FIDs
will be converted to use it later in the series.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>