Bluetooth: btmtksdio: Add a bit definition for CHLPCR

Add a register bit definition about CHLPCR bit 8 because the bit is quite
different in the meaning between reading and writing that bit.

The patch adds a definition particularly for the bit read to avoid the
confusion about using write definition to read the bit.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>