net/mlx5: fix mutex unlock in Tx packet pacing cleanup

net/mlx5: fix mutex unlock in Tx packet pacing cleanup

The lock sh->txpp.mutex was not correctly released on one path
of cleanup function return, potentially causing the deadlock.

Fixes: d133f4cdb706 ("net/mlx5: create clock queue for packet pacing")
Cc: stable@dpdk.org
Signed-off-by: Chengfeng Ye <cyeaa@connect.ust.hk>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>