Allow multiple 'log gsmtap' sinks

Previously we've checked for existing log target with a given hostname
from vty code but it was ignored inside the check so only the very first
'log gsmtap' entry was enabled while the rest were silently ignored.

Change-Id: I8fd8bda9e07d403a54735da30addb742e56538a2
diff --git a/src/logging.c b/src/logging.c
index 1e0b21b..e4f3e87 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -745,11 +745,18 @@
 	llist_for_each_entry(tgt, &osmo_log_target_list, entry) {
 		if (tgt->type != type)
 			continue;
-		if (tgt->type == LOG_TGT_TYPE_FILE) {
+		switch (tgt->type) {
+		case LOG_TGT_TYPE_FILE:
 			if (!strcmp(fname, tgt->tgt_file.fname))
 				return tgt;
-		} else
+			break;
+		case LOG_TGT_TYPE_GSMTAP:
+			if (!strcmp(fname, tgt->tgt_gsmtap.hostname))
+				return tgt;
+			break;
+		default:
 			return tgt;
+		}
 	}
 	return NULL;
 }