nak/bitset: Fix next_set()

It was resetting mask to 0 instead of u32::MAX.  This is a copy+paste
error from when I copied it from next_unset() and tweaked it to find set
bits.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24998>

src/nouveau/compiler/bitset.rs

@@ -74,7 +74,7 @@ impl BitSet {
             if b < 32 {
                 return Some(w * 32 + usize::try_from(b).unwrap());
             }
-            mask = 0;
+            mask = u32::MAX;
             w += 1;
         }
         None