Bluetooth: Fix cmd_status handling for status == 0

For commands that generate an intermediate zero status (e.g.
HCI_LE_Create_Connection) we should allow for successful
bt_cmd_send_sync() completion at this point. Since the code treats a
NULL return buffer as failure we must provide an actual buffer to
indicate success. There's not really any useful information in the buf
for a cmd_status but passing it to the caller lets us notify of the
successful completion.

Change-Id: I38c70bfce95a1cc5383dfa0a49903731705772d1
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>