usb: gadget: fsl_udc_core: fix ep valid checks

Clang reports the following warning:
  drivers/usb/gadget/udc/fsl_udc_core.c:1312:10: warning: address of array
  'ep->name' will always evaluate to 'true' [-Wpointer-bool-conversion]
        if (ep->name)
        ~~  ~~~~^~~~

It seems that the authors intention was to check if the ep has been
configured through struct_ep_setup. Check whether struct usb_ep name
pointer has been set instead.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>