usb: otg: gpio_vbus: Start using struct usb_otg

Use struct usb_otg members with OTG specific functions instead
of usb_phy members.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>