bng_en: periodically fetch and accumulate hardware statistics

bng_en: periodically fetch and accumulate hardware statistics

Use the timer to schedule periodic stats collection via
the workqueue when the link is up. Fetch fresh counters from
hardware via DMA and accumulate them into 64-bit software
shadows, handling wrap-around for counters narrower than
64 bits.

Signed-off-by: Bhargava Marreddy <bhargava.marreddy@broadcom.com>
Reviewed-by: Vikas Gupta <vikas.gupta@broadcom.com>
Reviewed-by: Rahul Gupta <rahul-rg.gupta@broadcom.com>
Reviewed-by: Ajit Kumar Khaparde <ajit.khaparde@broadcom.com>
Link: https://patch.msgid.link/20260406180420.279470-8-bhargava.marreddy@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>