Bluetooth: btmtk: move btusb_mtk_[setup, shutdown] to btmtk.c

Move btusb_mtk_[setup, shutdown] and related function from
btusb.c to btmtk.c which holds vendor specific stuff and
would make btusb.c clean.

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>