Bluetooth: host: Handle ATT timeout on disconnected ATT channel

Handle ATT timeout on disconnected ATT channel.
When the ATT channel is disconnected with a pending request the ATT
timeout is canceled and the response processed with an error code.
However canceling of delayed work is not guaranted to succeed, e.g:
"Work queue thread has removed the work item from the queue but has not
 called its handler"
This could lead to timeout handler being invoked after the disconnected
handler.

Fixes: #29098

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>