Bluetooth: btusb: mediatek: return error for failed reg access

Update the code to immediately return an error code if accessing a
related register fails. This ensures that our desired logic for
subsequent register operations is maintained and allows us to promptly
catch any unexpected errors.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Chris Lu <chris.lu@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>