Bluetooth: Hold ref on hci_conn when setting up A2MP fixed channel

Take a reference on the hci_conn and do not de-reference l2cap_conn
while setting up the A2MP fixed channel. l2cap_conn is not reference
counted and may go away before the channel is set up.

This fixes scenario where the ACL disconnects (and l2cap_conn goes
away) while amp_conn_ind worker is running or is on the workqueue
waiting to run.

Change-Id: I10fc6d9b146fcc5e010f26a046f7e0570f2b93dd
CRs-fixed: 347079
Signed-off-by: Peter Krystad <pkrystad@codeaurora.org>