usb: gadget: renesas_usbhs: check device0 status when alloc

device0 was treated without checking in usbhsh_device_alloc().
but "udev->usbv" and "dev_set_drvdata()" will be overwritten
if device0  was multi-allocated.
This patch fixes this issue.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>