gatt: Fix handling GATT connect timeout

When there is a timeout on create connection, all gatt clients
interested in the connection shall be notified about TIMEOUT.

With the recent changes improving creating multiple connections, there
is no TCB allocated for each connection we want to create. Therefore,
there is no unique conn_id allocated for the p_clcb.

That means, when notifing about timeout, stack shell not generate
CONN_ID using tcb_idx=0 as it could be already use by other device.
This could lead to situation that GATT thinks that other connected
devices just got disconnected.

Bug: 335816515
Bug: 325595120
Test: atest pts-bot:GATT
Change-Id: Ibf013b196caf38d46e81d6b97610060872697176