mlx5 devices were observed generating MLX5_PORT_CHANGE_SUBTYPE_ACTIVE
events without an intervening MLX5_PORT_CHANGE_SUBTYPE_DOWN. This
breaks link flap detection based on Linux carrier state transition
count as netif_carrier_on() does nothing if carrier is already on.
Make sure we count such events.

netif_carrier_event() increments the counters and fires the linkwatch
events. The latter is not necessary for the use case but seems like
the right thing to do.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>