Return network info error code

The error code from the core side was not correctly propagated to
user space.

Change-Id: I40e502f8dc3a77de064f441beeb62603d013e318
diff --git a/kernel/ethosu_network_info.c b/kernel/ethosu_network_info.c
index da95309..52adf9c 100644
--- a/kernel/ethosu_network_info.c
+++ b/kernel/ethosu_network_info.c
@@ -116,6 +116,8 @@
 		goto deregister;
 	}
 
+	ret = info->errno;
+
 deregister:
 	ethosu_mailbox_deregister(&info->edev->mailbox, &info->msg);
 	ethosu_network_put(info->net);