Bluetooth: btmtksdio: use register CRPLR to read packet length

That is a preliminary patch to introduce mt7921s support.

Use the register CRPLR to read packet length to make all the devices
share the common logic.

Co-developed-by: Mark-yw Chen <mark-yw.chen@mediatek.com>
Signed-off-by: Mark-yw Chen <mark-yw.chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>