logging vty: write: check logging levels validity

Change-Id: If683b1272ad2d5b402b91c52c496aa7975bc6315
diff --git a/src/logging.c b/src/logging.c
index 67470a5..e7cc472 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -63,7 +63,7 @@
 void *tall_log_ctx = NULL;
 LLIST_HEAD(osmo_log_target_list);
 
-static const struct value_string loglevel_strs[] = {
+const struct value_string loglevel_strs[] = {
 	{ LOGL_DEBUG,	"DEBUG" },
 	{ LOGL_INFO,	"INFO" },
 	{ LOGL_NOTICE,	"NOTICE" },