sku: Fix botched read_file check

<0 indicates failure.

BUG=b:214460179
TEST='mosys platform model' on delbin

Change-Id: I0323b456a78ee7e2c16d34918ba7a389a8e8d76b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mosys/+/3389531
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Auto-Submit: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
1 file changed