selftests: firmware: Fix ingonred return val of asprintf() warn

Fix the following ingonred return val of asprintf() warn during build:

cc -Wall -O2 fw_namespace.c -o ../tools/testing/selftests/firmware/fw_namespace
fw_namespace.c: In function ‘main’:
fw_namespace.c:132:2: warning: ignoring return value of ‘asprintf’ declared with attribute ‘warn_unused_result’ [-Wunused-result]

  132 |  asprintf(&fw_path, "/lib/firmware/%s", fw_name);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

---
 tools/testing/selftests/firmware/fw_namespace.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

index 0e393cb5f42d..4c6f0cd83c5b 100644
@@ -129,7 +129,8 @@ int main(int argc, char **argv)
 		die("mounting tmpfs to /lib/firmware failed\n");
 
 	sys_path = argv[1];
-	asprintf(&fw_path, "/lib/firmware/%s", fw_name);
+	if (asprintf(&fw_path, "/lib/firmware/%s", fw_name) < 0)
+		die("error: failed to build full fw_path\n");
 
 	setup_fw(fw_path);
 

2.30.2