net/mlx5: debugfs, Add num of in-use FW command interface slots

Expose the number of busy / in-use slots in the FW command interface via
a read-only debugfs entry. This improves observability and helps in the
performance bottleneck analysis.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>