usb: mtu3: get optional vbus for host only mode

When dr_mode is set as USB_DR_MODE_HOST, it's better to try to
get optional vbus, this can increase flexibility, although we
can set vbus as always on for regulator or put it in host driver
to turn it on.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>