net/mlx5: Remove redundant health work lock

Commit 90e7cb78 ("net/mlx5: fix missing mutex_unlock in
mlx5_fw_fatal_reporter_err_work()") introduced another checking of
MLX5_DROP_HEALTH_NEW_WORK. At this point, the first check of
MLX5_DROP_HEALTH_NEW_WORK is redundant and so is the lock that
protects it.

Remove the lock and rename MLX5_DROP_HEALTH_NEW_WORK to reflect these
changes.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>