bng_en: add per-PF workqueue, timer, and slow-path task

bng_en: add per-PF workqueue, timer, and slow-path task

Add a dedicated single-thread workqueue and a timer for each PF
to drive deferred slow-path work such as link event handling and
stats collection. The timer is stopped via timer_delete_sync()
when interrupts are disabled and restarted on open.

While the close path stops the timer to prevent new tasks from
being scheduled, the sp_task and workqueue are preserved to
maintain state continuity. Final draining and destruction of
the workqueue are handled during PCI remove.

Signed-off-by: Bhargava Marreddy <bhargava.marreddy@broadcom.com>
Reviewed-by: Vikas Gupta <vikas.gupta@broadcom.com>
Reviewed-by: Ajit Kumar Khaparde <ajit.khaparde@broadcom.com>
Link: https://patch.msgid.link/20260406180420.279470-2-bhargava.marreddy@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>