Bluetooth: btusb: mediatek: refactor the function btusb_mtk_reset

Extract the function btusb_mtk_subsys_reset from the btusb_mtk_reset
for the future handling of resetting bluetooth controller without
the USB reset.

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Hao Qin <hao.qin@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>