mlxsw: spectrum_router: Add missing rollback

With this patch the error path of mlxsw_sp_nexthop_init() is symmetric
with mlxsw_sp_nexthop_fini(). Noticed during code review.

Fixes: a8c97014 ("mlxsw: spectrum_router: Refactor nexthop init routine")
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>