fix(prim): Lint warning for `err_o`

This commit fixes lint warning for undriven `err_o` in case of `Depth ==
0`.

Signed-off-by: Eunchan Kim <eunchan@opentitan.org>

hw/ip/prim/rtl/prim_fifo_sync.sv

diff --git a/hw/ip/prim/rtl/prim_fifo_sync.sv b/hw/ip/prim/rtl/prim_fifo_sync.sv
index 1f2115d..943ff06 100644
--- a/hw/ip/prim/rtl/prim_fifo_sync.sv
+++ b/hw/ip/prim/rtl/prim_fifo_sync.sv
@@ -51,6 +51,9 @@
     logic unused_clr;
     assign unused_clr = clr_i;
 
+    // No error
+    assign err_o = 1'b 0;
+
   // Normal FIFO construction
   end else begin : gen_normal_fifo