This factors out a usb_setup_descriptor() function with the goal to
make the code better comparable to the corresponding U-Boot code.

This also incorporates this U-Boot commit:

| commit 2b338ef
| Author: Hans de Goede <hdegoede@redhat.com>
| Date:   Tue May 5 23:56:04 2015 +0200
|
|     usb: Fix maxpacketsize for first descriptor read for low-speed usb devs
|
|     This fixes descriptor reading of lowspeed devices through ohci not working.
|
|     Signed-off-by: Hans de Goede <hdegoede@redhat.com>
|     Acked-by: Marek Vasut <marex@denx.de>

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>