net: dsa: ocelot: unlock on error in vsc9959_qos_port_tas_set()

commit cad7526f upstream.

This error path needs call mutex_unlock(&ocelot->tas_lock) before
returning.

Fixes: 2d800bc5

 ("net/sched: taprio: replace tc_taprio_qopt_offload :: enable with a "cmd" enum")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>