buffet: fix signed warnings

Newer gcc complains about signed mismatch like so:
error: comparison between signed and unsigned integer expressions [

BUG=chromium:423463
TEST=None

Change-Id: I8ab4a4c834f5972c66ed7cbfc54e9648bcfb201f
Reviewed-on: https://chromium-review.googlesource.com/230011
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>