debug: Make the global target loglevel overwrite the category level

Together with the previos -e Number option one can easily turn on
debugging without needing to remember the category masks for a quick
check on what is going on.
diff --git a/openbsc/src/debug.c b/openbsc/src/debug.c
index 3bb5309..8f7a1c3 100644
--- a/openbsc/src/debug.c
+++ b/openbsc/src/debug.c
@@ -242,7 +242,7 @@
 			continue;
 
 		/* Check the category log level */
-		if (category->loglevel != 0 && level < category->loglevel)
+		if (tar->loglevel == 0 && category->loglevel != 0 && level < category->loglevel)
 			continue;
 
 		/*