mlxsw: spectrum_router: Set ifindex for IPv4 nexthops

The ifindex of the nexthop device was never set for IPv4 nexthops,
unlike IPv6 nexthops. This went unnoticed since only IPv6 nexthops use
it.

Set the ifindex for IPv4 nexthops in order to be consistent with IPv6
and also because it will be used by a later patch.

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