Fix earfcn deletion

* fix typo in arg index
* fix sign in error reporting
* add vty test
diff --git a/openbsc/src/libbsc/bsc_vty.c b/openbsc/src/libbsc/bsc_vty.c
index 3a699c4..85b8394 100644
--- a/openbsc/src/libbsc/bsc_vty.c
+++ b/openbsc/src/libbsc/bsc_vty.c
@@ -2802,11 +2802,11 @@
 {
 	struct gsm_bts *bts = vty->index;
 	struct osmo_earfcn_si2q *e = &bts->si_common.si2quater_neigh_list;
-	uint16_t arfcn = atoi(argv[1]);
+	uint16_t arfcn = atoi(argv[0]);
 	int r = osmo_earfcn_del(e, arfcn);
 	if (r < 0) {
 		vty_out(vty, "Unable to delete arfcn %u: %s%s", arfcn,
-			strerror(r), VTY_NEWLINE);
+			strerror(-r), VTY_NEWLINE);
 		return CMD_WARNING;
 	}