mlxsw: spectrum_router: Compare key with correct object type

When comparing a key with a nexthop group in rhastable's obj_cmpfn()
callback, make sure that the key and nexthop group are of the same type
(i.e., IPv4 / IPv6).

The bug is not currently visible because IPv6 nexthop groups do not
populate the FIB info pointer and IPv4 nexthop groups do not set the
ifindex for the individual nexthops.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>