up_clock_mon: Explicitly truncate d_count during up_d_count assignment

The MSB of the d_count signal is used as a overflow marker to stop the
counter from incrementing in the monitored clock domain. It is not exported
through the register map and truncated when assigned to the up_d_count
signal.

Make the truncation explicit to make it clear that this is not a mistake
and to avoid warnings about implicit truncation.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

library/common/up_clock_mon.v

@@ -96,7 +96,7 @@ module up_clock_mon #(
       end
 
       if (up_count_capture_s == 1'b1) begin
-        up_d_count <= d_count;
+        up_d_count <= d_count[TOTAL_WIDTH-1:0];
       end else if (up_count == 'h00 && up_count_run != up_count_running_m3) begin
         up_d_count <= 'h00;
       end