fixes according to review

stan/math/opencl/kernels/sub_block.hpp

@@ -47,13 +47,13 @@ static const char *sub_block_kernel_code = STRINGIFY(
         unsigned int dst_offset_j, unsigned int size_i, unsigned int size_j,
         unsigned int src_rows, unsigned int src_cols, unsigned int dst_rows,
         unsigned int dst_cols, unsigned int partial_view) {
-      int i = get_global_id(0);
-      int j = get_global_id(1);
+      const int i = get_global_id(0);
+      const int j = get_global_id(1);
 
-      int src_idx_i = i + src_offset_i;
-      int src_idx_j = j + src_offset_j;
-      int dst_idx_i = i + dst_offset_i;
-      int dst_idx_j = j + dst_offset_j;
+      const int src_idx_i = i + src_offset_i;
+      const int src_idx_j = j + src_offset_j;
+      const int dst_idx_i = i + dst_offset_i;
+      const int dst_idx_j = j + dst_offset_j;
 
       if (src_idx_i < src_rows && src_idx_j < src_cols && dst_idx_i < dst_rows
           && dst_idx_j < dst_cols) {

stan/math/opencl/sub_block.hpp

@@ -62,14 +62,14 @@ inline void matrix_cl<T, enable_if_arithmetic<T>>::sub_block(
                               this->rows(), this->cols(), A.partial_view());
   }
   // calculation of extreme sub- and super- diagonal written
-  int diag_in_copy = A_i - A_j;
-  int copy_low = is_not_diagonal(A.partial_view(), PartialViewCL::Lower)
+  const int diag_in_copy = A_i - A_j;
+  const int copy_low = is_not_diagonal(A.partial_view(), PartialViewCL::Lower)
                      ? 1 - nrows
                      : diag_in_copy;
-  int copy_high = is_not_diagonal(A.partial_view(), PartialViewCL::Upper)
+  const int copy_high = is_not_diagonal(A.partial_view(), PartialViewCL::Upper)
                       ? ncols - 1
                       : diag_in_copy;
-  int start = this_j - this_i;
+  const int start = this_j - this_i;
 
   if (start + copy_low < 0) {
     this->partial_view_ = this->partial_view_ + PartialViewCL::Lower;