usb: gadget: regression fix - usage of usb_ep

This patch removes redundant pointer to struct usb_endpoint_descriptor which
were missed in commit 79149b8b:

 usb: gadget: Update fsl_udc_core to use usb_endpoint_descriptor inside the
 struct usb_ep

Due to clock framework regressions, this patch is only compile tested!

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>