Bluetooth: hci_sync: make hci_cmd_sync_run_once return -EEXIST if exists

Bluetooth: hci_sync: make hci_cmd_sync_run_once return -EEXIST if exists

hci_cmd_sync_run_once() needs to indicate whether a queue item was
added, so caller can know if callbacks are called, so it can avoid
leaking resources.

Change the function to return -EEXIST if queue item already exists.

Modify all callsites vs. the changes.  The only callsite is
hci_abort_conn().

Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>