usb: gadget: udc: renesas_usb3: protect usb3_ep->started in usb3_start_pipen()

This patch fixes an issue that unexpected behavior happens when
both the interrupt handler and renesas_usb3_ep_enable() are called.
In this case, since usb3_start_pipen() checked the usb3_ep->started,
but the flags was not protected. So, this patch protects the flag
by usb3->lock. Since renesas_usb3_ep_enable() for EP0 will be not called,
this patch doesn't take care of usb3_start_pipe0().

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>