Fix warning 186-D from nvcc (#2527)

mlx/backend/cuda/device/binary_ops.cuh

@@ -205,9 +205,11 @@ struct Power {
     if constexpr (cuda::std::is_integral_v<T>) {
       T res = 1;
       // Raising an integer to a negative power is undefined
+      if constexpr (cuda::std::is_signed_v<T>) {
         if (exp < 0) {
           return 0;
         }
+      }
       while (exp) {
         if (exp & 1) {
           res *= base;