usb: mtu3: add helper to get pointer of ssusb_mtk struct

Add a helper to get pointer of ssusb_mtk struct from the pointer
of otg_switch_mtk struct.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1623139069-8173-12-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>