module: demote ELF structure error message to pr_debug

This happens anytime busybox modprobe loads a compressed module and
looks especially ugly on postmarketOS for example (or basically any
initramfs); this error is rather pointless since the module is loaded
anyway afterwards.

Upstream (busybox) has no plans to do anything about this error getting
spammed:
https://www.mail-archive.com/busybox@busybox.net/msg27078.html

kernel/module.c

@@ -3941,7 +3941,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
 	 */
 	err = elf_validity_check(info);
 	if (err) {
-		pr_err("Module has invalid ELF structures\n");
+		pr_debug("Module has invalid ELF structures\n");
 		goto free_copy;
 	}