usb: Alloc buffer for USB descriptor dynamically

The configuration descriptor includes all interface, endpoint and
auxiliary descriptors (e.g. report, union) so 512 bytes may not be enough.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>