hw: Add missing cast in ambiguous default expression (#147)

* hw: fix slang casting error

Slang says:
error: assignment pattern target type cannot be deduced in this context

According to the spec chapter 6.24.1 the parenthesis
around the expression to be casted are necessary.
This fixes the slang error.

* hw: fix ambiguous inferred type in default

Its not pretty but it works.
Without the explicit cast or type slang is unable to infer
the type of the value in the default.
By just adding an explicit cast parenthesis like: '('{0, 0})
Vivado gives a Syntax error.

So adding a localparam seems to be the simplest fix.

* hw: fix verible linter warning

* Improve naming, add clarifying comment on default case

---------

Co-authored-by: Paul Scheffler <[email protected]>

hw/cheshire_pkg.sv

@@ -463,6 +463,7 @@ package cheshire_pkg;
 
   // Choose static colocation of IDs based on how heavily used and/or critical they are
   function automatic cva6_id_map_t gen_cva6_id_map(cheshire_cfg_t cfg);
+    localparam int unsigned DefaultMapEntry[2] = '{0, 0};
     case (cfg.AxiMstIdWidth)
       // Provide exclusive ID to I-cache to prevent fetch blocking
       1: return '{'{Cva6IdBypMmu, 0}, '{Cva6IdBypLoad, 0}, '{Cva6IdBypAccel, 0}, '{Cva6IdBypStore, 0},
@@ -473,8 +474,9 @@ package cheshire_pkg;
       // Compress output ID space without any serialization
       3: return '{'{Cva6IdBypMmu, 0}, '{Cva6IdBypLoad, 1}, '{Cva6IdBypAccel, 6}, '{Cva6IdBypStore, 2},
                   '{Cva6IdBypAmo, 3}, '{Cva6IdICache,  4}, '{Cva6IdDCache,   5}};
-      // With 4b of ID or more, no remapping is necessary
-      default: return '{default: '{0, 0}};
+      // With 4b of ID or more, no remapping is necessary; return redundant 0 -> 0 ID remaps.
+      // This leaves ID mapping unaltered only if `MstIdBaseOffset` in `axi_id_serialize` is 0.
+      default: return '{default: DefaultMapEntry};
     endcase
   endfunction