usb: phy: generic: remove the vbus dependency

As the last known user, ie. pxa27x_udc relying on calls to
usb_gadget_xxx() was amended to use the phy notifier, remove a bit the
USB stack adherence.

Actually the driver still uses the gadget API for structures definition,
but the implementation of USB gadget specific function usb_gadget_*() is
not necessary anymore.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>