usb: renesas_usbhs: gadget: fix unused-but-set-variable warning

The commit b8b9c974 ("usb: renesas_usbhs: gadget: disable all eps
when the driver stops") causes the unused-but-set-variable warning.
But, if the usbhsg_ep_disable() will return non-zero value, udc/core.c
doesn't clear the ep->enabled flag. So, this driver should not return
non-zero value, if the pipe is zero because this means the pipe is
already disabled. Otherwise, the ep->enabled flag is never cleared
when the usbhsg_ep_disable() is called by the renesas_usbhs driver first.

Fixes: b8b9c974 ("usb: renesas_usbhs: gadget: disable all eps when the driver stops")
Fixes: 11432050 ("usb: renesas_usbhs: gadget: fix NULL pointer dereference in ep_disable()")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>