Bluetooth: hci_sync: fix undefined return of hci_disconnect_all_sync()

clang static analysis reports this problem
hci_sync.c:4428:2: warning: Undefined or garbage value
  returned to caller
        return err;
        ^~~~~~~~~~

If there are no connections this function is a noop but
err is never set and a false error could be reported.
Return 0 as other hci_* functions do.

Fixes: 182ee45d ("Bluetooth: hci_sync: Rework hci_suspend_notifier")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>