Bluetooth: btmtk: move btusb_mtk_hci_wmt_sync to btmtk.c

Move btusb_mtk_hci_wmt_sync from btusb.c to btmtk.c which holds
vendor specific stuff and would make btusb.c clean.

Add usb.h header to btmtksdio.c/btmtkuart.c for usb related element
defined in btmtk.h

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>