ice40/pack: Stop looking for BEL when we have one during PLL placement

Ideally we should first process all the PLL that are constrained somehow
(either explicitely or because they are PAD) and then free place the rest.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

ice40/pack.cc

@@ -881,6 +881,7 @@ static void pack_special(Context *ctx)
                     packed->attrs[ctx->id("BEL")] = ctx->getBelName(bel).str(ctx);
                     pll_bel = bel;
                     constrained = true;
+                    break;
                 }
                 if (!constrained) {
                     log_error("Could not constrain PLL '%s' to any PLL Bel (too many PLLs?)\n",