[kmac] Fix AscentLint errors

Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>
diff --git a/hw/ip/kmac/rtl/kmac_entropy.sv b/hw/ip/kmac/rtl/kmac_entropy.sv
index 13edc52..3188c4d 100644
--- a/hw/ip/kmac/rtl/kmac_entropy.sv
+++ b/hw/ip/kmac/rtl/kmac_entropy.sv
@@ -352,7 +352,7 @@
   );
 
   assign lfsr_seed_done =
-      (seed_idx == SeedIdxWidth'(NumSeedsEntropyLfsr - 1)) &
+      (seed_idx == SeedIdxWidth'(unsigned'(NumSeedsEntropyLfsr - 1))) &
       |lfsr_seed_en;
 
   // Seed selection - The reduced seed enable signal `lfsr_seed_en_red` is