Bluetooth: Fix memory leak in hci_req_sync_complete()

In 'hci_req_sync_complete()', always free the previous sync
request state before assigning reference to a new one.

Reported-by:  <syzbot+39ec16ff6cc18b1d066d@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=39ec16ff6cc18b1d066d
Cc: stable@vger.kernel.org
Fixes: f60cb305

 ("Bluetooth: Convert hci_req_sync family of function to new request API")
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>