mlxsw: spectrum_router: Implement common RIF core

The mlxsw driver currently implements three types of RIFs. VLAN and FID
RIFs for L3 interfaces on top of VLAN-aware and VLAN-unaware bridges
(respectively) and Subport RIFs for all other L3 interfaces.

All the RIF types follow a common configuration procedure, which only
differs in the type-specific bits. The patch exploits this fact and
consolidates the common code paths, thereby simplifying the code and
making it more extensible.

This work also prepares the driver for use with future ASICs, where the
range of the Subport RIFs will be extended and their configuration
modified accordingly. By merely implementing a new RIF operations and
selecting it during initialization, the same driver could be re-used.

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>