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 <[email protected]>
Signed-off-by: Chris Lu <[email protected]>
Signed-off-by: Luiz Augusto von Dentz <[email protected]>

Author: ChrisCH-Lu