Bluetooth: mt7921s: Fix the incorrect pointer check

Fix the incorrect pointer check on ven_data.

Fixes: f41b91fa ("Bluetooth: mt7921s: Add .btmtk_get_codec_config_data")
Co-developed-by: Yake Yang <yake.yang@mediatek.com>
Signed-off-by: Yake Yang <yake.yang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>