usb: dwc2: disable external vbus supply when the device is removed

This patch adds an interface to disable the power in dwc2 driver.
This new interface is called when the device is removed.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>