Bluetooth: host: clear ATT_CONNECTED flag on detach

Clear the `ATT_CONNECTED` flag when a channel is detached (could be after
an ATT timeout).

Also convert the assert checking it in `chan send` to an `if` test, since
the channel could be disconnected from a different thread than the one
triggering `chan_send`.

Fixes #53247.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>