usb: dwc2: gadget: Check for ep0 in enable

Replaced the WARN_ON with a check and return of -EINVAL in the
dwc2_hsotg_ep_enable function if ep0 is passed in.

Signed-off-by: Vahram Aharonyan <vahrama@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>