usb_gadget_connect — software-controlled connect to USB host
int usb_gadget_connect (struct usb_gadget * gadget);
Enables the D+ (or potentially D-) pullup. The host will start
enumerating this gadget when the pullup is active and a VBUS session
is active (the link is powered). This pullup is always enabled unless
usb_gadget_disconnect has been used to disable it.
Returns zero on success, else negative errno.