mlxsw: spectrum_router: Return -ENOENT in case of error

'vr' should be a valid pointer here, so returning 'PTR_ERR(vr)' is wrong.
Return an explicit error code (-ENOENT) instead.

Fixes: 61c503f9 ("mlxsw: spectrum_router: Implement fib4 add/del switchdev obj ops")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>