net/mlx5: Avoid exposing driver internal command helpers

mlx5 command init and cleanup routines are internal to mlx5_core driver.
Hence, avoid exporting them and move their definition to mlx5_core
driver's internal file mlx5_core.h

Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>