mlxsw: spectrum_mr: Convert into using mr_mfc

Current multicast routing logic in driver assumes it's always meant to
deal with IPv4 multicast routes, leaving several placeholders for
later IPv6 support [currently usually WARN()].

This patch changes the driver's internal multicast route struct into
holding a common mr_mfc instead of the IPv4 mfc_cache.
The various placeholders are grouped into 2:
  - Functions that require only the common bits; These remain and the
    restriction for IPv4-only is lifted.
  - Function that require IPv4-specifics - for handling these functions
    we add sets of operations that encapsulate the protocol differences
Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>