Bluetooth: Host: L2CAP: Don't send credits if chan is disconnected

Fixes bug where Host sends L2CAP Flow Control Credit after receiving a
L2CAP Disconnect Response.

The when the callback in l2cap_chan_le_recv_sdu is handed a Disconnect
Reponse, the connnection becomes disconnected, as illustrated by the
asserts. The function should now trigger sending credits if the
connection is disconnected after the callback returns.

Fixes #42112

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>