Bluetooth: btmtk: rename btmediatek_data

Rename btmediatek_data to have a consistent prefix throughout the driver.

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>