L2CAP: Discard pending requests on link disconnect

Scenario: A connection request packet is admitted to link tx queue in
L2CAP but before it could be transmitted (host busy), ACL link can get
disconnected (idle timeout) and the link tx queue is not flushed during
link disconnect handling. In such scenario, subsequent connection
attempt by host is seen to fail. This is because host is found to send
multiple connection requests (one connect request packet from the queue
and a new connection request packet per new request). Remote accepts old
connection request packet and waits for connection response from host.
For next connection request seen by remote, remote responds with 'No
PSM' resource error (as it already admitted one connection request and
it is in progress). Here when the connection response arrives from
remote, host ignores it because it sees packet id mismatch (host had
sent old connection request first and later information request/response
packets were exchanged. Remote had sent connection response copying
stale packet id picked from old connection request).

Drop queued packets as part of link disconnection handling.
This was already done in BLE transport case and is now being done
unconditionally.

Fixes: 73050475
Test: back-to-back PAN connect-disconnect iterations amidst ongoing
      OPP file transfer and AV streaming
Change-Id: I42cc7ae90cc1152f1781282698529fe50f0ebf89