vty: 'hopping arfcn add': succeed if adding arfcn already in set

There's no need to fail, simply make it a noop in that case,
everything's fine and everybody is happy (specially when using CTRL
command "apply-config-file" to load a .cfg file containing
modifications.

Related: SYS#6138
Change-Id: Ia4e70d20d48a28c46a21dd10358577e5c798744c

src/osmo-bsc/bts_trx_vty.c

@@ -402,7 +402,7 @@ DEFUN_USRATTR(cfg_ts_arfcn_add,
 
 	if (bitvec_get_bit_pos(&ts->hopping.arfcns, arfcn) == ONE) {
 		vty_out(vty, "%% ARFCN %" PRIu16 " is already set%s", arfcn, VTY_NEWLINE);
-		return CMD_WARNING;
+		return CMD_SUCCESS;
 	}
 
 	bitvec_set_bit_pos(&ts->hopping.arfcns, arfcn, 1);