usb: gadget: move non-super speed code out of usb_ep_autoconfig_ss()

The moved code refers to non-super speed endpoints only. This patch also
makes the comment stress the fact, that autoconfigured descriptor might
need some adjustments.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>