usb: sandbox: Bounds check read from buffer

The buffer is 512 bytes but read requests can be 800 bytes. Limit the
request to the size of the buffer.

Signed-off-by: Andrew Scull <ascull@google.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>