usb: dwc2: Fix inefficient copy of unaligned buffers

Make sure only to copy any actual data rather than the whole buffer,
when releasing the temporary buffer used for unaligned non-isochronous
transfers.

Taken directly from commit 0efd937e ("USB: ehci-tegra: fix inefficient
copy of unaligned buffers")

Tested with Lantiq xRX200 (MIPS) and RPi Model B Rev 2 (ARM)

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Antti Seppälä <a.seppala@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>