Before, it would alloc the endpoint fine and then panic later due to out of range.
This ensures it falis at ep alloc time, and with a panic message that says
what's the actual problem: "no free endpoints available".
This makes location info in defmt logs point to the code calling the macro,
instead of always to fmt.rs as before. Fix works with nightlies
starting with today's, and stable 1.81+.
This restores the Send/Sync impls for EpState, which were incidentally
removed in #2881. This allows the Endpoints to be send to tasks, e.g.
as part of a custom class implementation.
