net/mlx5: fix missing mutex_unlock in mlx5_fw_fatal_reporter_err_work()

Add missing mutex_unlock() before returning from
mlx5_fw_fatal_reporter_err_work().

Fixes: 9078e843 ("net/mlx5: Avoid recovery in probe flows")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>