cxgb4: fix error return code in cxgb4_set_hash_filter()

Fix to return a negative error code from thecxgb4_alloc_atid()
error handling case instead of 0.

Fixes: 12b276fb ("cxgb4: add support to create hash filters")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-By: Kumar Sanghvi <kumaras@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>