devlink: fix return value check in devlink_dpipe_header_put()

Fix the return value check which testing the wrong variable
in devlink_dpipe_header_put().

Fixes: 1555d204 ("devlink: Support for pipeline debug (dpipe)")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>