usb: mtu3: use enum usb_role instead of private defined ones

Now we mainly use usb-role-switch to set dual role mode, and all
three ways supported use the same function to switch mode, use
usb_role enum will make code clear

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1623139069-8173-10-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>