usb: dwc2: avoid out of bounds access

flush_dcache_range may access data after priv->aligned_buffer end if
len > DWC2_DATA_BUF_SIZE.
memcpy may access data after buffer end if done > 0

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>