mlxsw: Split handling of FDB tunnel entries between address families

Currently, the function which adds/removes unicast tunnel FDB entries is
shared between IPv4 and IPv6, while for IPv6 it warns because there is
no support for it.

The code for IPv6 will be more complicated because it needs to
allocate/release a KVDL pointer for the underlay IPv6 address.

As a preparation for IPv6 underlay support, split the code according to
address family.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>