[bkdr_loader,rtl] Fix tgt_idx_err

tgt_idx_err was not set when the index exceeded NumBkdrTgts because it
was casted to a logic with fewer bits. The enum is now changed to an 8b
enum such that the cast is not dropping any bits.

Signed-off-by: Michael Gautschi <mgautschi@lowrisc.org>

Author: gautschimi

hw/ip/bkdr_loader/rtl/bkdr_loader_pkg.sv

@@ -31,7 +31,7 @@ package bkdr_loader_pkg;
   typedef logic [MaxWordWidth-1:0] word_t;
 
   // Target indices
-  typedef enum logic [$clog2(NumBkdrTgts)-1:0] {
+  typedef enum logic [7:0] {
     BkdrAon       = 'd11,
     BkdrFlashB1I2 = 'd10,
     BkdrFlashB1I1 = 'd9,