net: bootp: Correct VCI string transmission

The VCI string sent during bootp of U-Boot-SPL is corrupt. This is
because the byte counter is not adjusted within the bootp_extended()
function when the VCI string is added. We fix this.

Signed-off-by: Walter Stoll <walter.stoll@duagon.com>
Reviewed-by: Simon Glass <sjg@chromium.org>