[prim_fifo_sync,rtl] Fix full_o behaviour when Depth == 0

This is a bit silly, but the storage of an empty fifo is always full
for rather trivial reasons. The existing connection didn't really make
sense (because it was considered full if something was able to read
from it!) so I'm pretty certain this won't break anything.

Signed-off-by: Rupert Swarbrick <[email protected]>

hw/ip/prim/rtl/prim_fifo_sync.sv

@@ -46,7 +46,7 @@ module prim_fifo_sync #(
 
     // host facing
     assign wready_o = rready_i;
-    assign full_o = rready_i;
+    assign full_o = 1'b1;
 
     // this avoids lint warnings
     logic unused_clr;