log: properly return -EINVAL from log_set_max_level_from_string()

If we just return the value we got from log_level_from_string() on
failure we'll return -1, which is not a proper error code.

log_set_target_from_string() did get this right already, hence let's fix
this here too.

src/basic/log.c

@@ -922,7 +922,7 @@ int log_set_max_level_from_string(const char *e) {
 
         t = log_level_from_string(e);
         if (t < 0)
-                return t;
+                return -EINVAL;
 
         log_set_max_level(t);
         return 0;