Fix frame FIFO drop

Author: alexforencich

rtl/axis_async_fifo.v

@@ -280,7 +280,7 @@ end
 always @* begin
     write = 1'b0;
 
-    drop_frame_next = 1'b0;
+    drop_frame_next = drop_frame_reg;
     overflow_next = 1'b0;
     bad_frame_next = 1'b0;
     good_frame_next = 1'b0;

rtl/axis_fifo.v

@@ -204,7 +204,7 @@ assign status_good_frame = good_frame_reg;
 always @* begin
     write = 1'b0;
 
-    drop_frame_next = 1'b0;
+    drop_frame_next = drop_frame_reg;
     overflow_next = 1'b0;
     bad_frame_next = 1'b0;
     good_frame_next = 1'b0;