shill: Fix a 'sign-compare' warning from clang. BUG=chromium:386185 TEST=`P2_TEST_FILTER='shill::*' USE='clang asan' FEATURES=test emerge-$BOARD platform2` Change-Id: I330ec198e14410dfb74b649c3b0abbfb689043b7 Reviewed-on: https://chromium-review.googlesource.com/204457 Tested-by: Ben Chan <benchan@chromium.org> Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org> Reviewed-by: Todd Broch <tbroch@chromium.org> Commit-Queue: Ben Chan <benchan@chromium.org>