ec/google/chromeec: fix the error status passing

Various instances of google_chromeec_command() can return non-zero number
(both positive and negative) to indicate error -- fixing cbi_get_uint32()
and cbi_get_string() so they follow the same convention.

BUG=b:123676982
BRANCH=kukui
TEST=build with kukui/flapjack configurations
Signed-off-by: YH Lin <yueherngl@google.com>
Change-Id: I7f0a8a61d01d942cba57036a17dd527fdbbf940c
Reviewed-on: https://review.coreboot.org/c/31585
Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/ec/google/chromeec/ec.c b/src/ec/google/chromeec/ec.c
index d2a0849..09357b0 100644
--- a/src/ec/google/chromeec/ec.c
+++ b/src/ec/google/chromeec/ec.c
@@ -595,7 +595,7 @@
 	cmd.cmd_dev_index = 0;
 
 	rv = google_chromeec_command(&cmd);
-	if (rv < 0)
+	if (rv != 0)
 		return rv;
 	*id = r;
 	return 0;
@@ -627,7 +627,7 @@
 	int rv;
 
 	rv = google_chromeec_command(&cmd);
-	if (rv < 0)
+	if (rv != 0)
 		return rv;
 
 	/* Ensure NUL termination. */