Bluetooth: SCO: fix sco_conn refcounting on sco_conn_ready

sco_conn refcount shall not be incremented a second time if the sk
already owns the refcount, so hold only when adding new chan.

Add sco_conn_hold() for clarity, as refcnt is never zero here due to the
sco_conn_add().

Fixes SCO socket shutdown not actually closing the SCO connection.

Fixes: ed958855 ("Bluetooth: SCO: remove the redundant sco_conn_put")
Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>