usb: musb: gadget: use platform callback to enable vbus

On some platform configurations (like OMAP3+twl4030) it's the platform
code that enables VBUS, not OTG transceiver, so call vbus platform
callback instead, it will then call the transceiver if needed.

This fixes a use case where USB cable is plugged first and gadget
driver is loaded later after that.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>