net/mlx5: Refactor completion irq request/release code

Break the request and release functions into pci and sub-functions
devices handling for better readability, eventually making the code
symmetric in terms of request/release.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>