Bluetooth: mt7921s: Add .get_data_path_id

Add .get_data_path_id to fetch data_path_id for MT7921 to support HFP
offload use case.

This is a preliminary patch to add the WBS support to the MT7921 driver.

Reviewed-by: Mark Chen <markyawenchen@gmail.com>
Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Yake Yang <yake.yang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>