L2C: Refactor l2c_link_hci_disc_comp

1. Release ccb before pL2CA_FixedConn_Cb because callbacks may release
   them as well.
2. Release ACL connection handles when reconnecting to avoid any usage
   of them before next connected.

Bug: 272193402
Test: net_test_stack_l2cap
Test: Avatar - standard pairing
Change-Id: I873b0e041183bddfab8c8b14d564046695b6906a