net: dsa: ksz9477: port mirror sniffing limited to one port

This patch limits the sniffing to only one port during the mirror add.
And during the mirror_del it checks for all the ports using the sniff,
if and only if no other ports are referring, sniffing is disabled.
The code is updated based on the review comments of LAN937x port mirror
patch.

Link: https://patchwork.kernel.org/project/netdevbpf/patch/20210422094257.1641396-8-prasanna.vengateshan@microchip.com/


Fixes: b987e98e ("dsa: add DSA switch driver for Microchip KSZ9477")
Signed-off-by: Prasanna Vengateshan <prasanna.vengateshan@microchip.com>
Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Link: https://lore.kernel.org/r/20220428070709.7094-1-arun.ramadoss@microchip.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>