mlxsw: spectrum_router: Mark IPv4 specific function accordingly

The functions to create and destroy a nexthop group are IPv4 specific
and should be renamed accordingly, so that they won't be confused with
the IPv6 specific functions in follow-up patches.

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>