[aes,dv] Add a cast for the byte type return of getc() function to st…

…ring variable assignment

Assign the result of getc function of a byte type to the string variable requires type casting.
Signed-off-by: Dariusz Stachanczyk <[email protected]>

hw/ip/aes/dv/env/seq_lib/aes_base_vseq.sv

@@ -503,7 +503,7 @@ class aes_base_vseq extends cip_base_vseq #(
 
   // the index of multi-reg is at the last char of the name
   virtual function int get_multireg_idx(string name);
-    string s = name.getc(name.len - 1);
+    string s = string'(name.getc(name.len - 1));
     return s.atoi();
   endfunction