ncm: fix carrier lost on link-state notify collision

tud_network_link_state() delivered the NETWORK_CONNECTION notification
edge-triggered and fire-once: if a previous notification was still in
flight, notification_xmit() returned early and the notification for the
new link state was never queued. Because link_is_up is committed before
the send, the host could be left reporting a stale carrier state - e.g.
a permanent NO-CARRIER after a link up. The notification state was also
mutated from both the caller and the notify xfer-completion callback
with no serialisation, so on RTOS ports where tud_network_link_state()
runs in a task other than tud_task() the two could race.

Defer the whole link-state update onto the usbd task, so it can no
longer race the completion callback. A collision with an in-flight
notification is resolved by re-arming notification_xmit_state and
letting the existing completion callback drive it forward on the next
xfer completion, rather than adding a separate pending/retry flag.

A link toggle does not change the link speed, so strictly only the
NETWORK_CONNECTION notification needs (re)sending, but reusing the
existing speed-then-connection state machine keeps the fix on a single,
already-serialised code path.

Closes #3760