device: stop ac6 grace time when ip6ll is ready in shared mode
The IPv6 shared mode starts IPv6 autoconf to send router advertisements. IPv6 autoconf schedules a 30-second timeout waiting for a link-local address to appear. When the link-local address appears, we need to cancel the timeout.
Fixes: 58287cbc ('core: rework IP configuration in NetworkManager using layer 3 configuration')