usb: Use the first available device for ehci_gadget

For whatever reason, usb_setup_ehci_gadget removes and probes USB device
0. However, not all systems have a device 0. Use the first device
instead.

The device probed should probably have something to do with the
controller (as specified by e.g. ums <controller> or fastboot
<controller>). In fact, I find it odd that we probe the USB device in
the first place, because this is just to set up the gadget itself.
Presumably, the controller should be probed by usb_gadget_initialize
somehow.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>