[uart] Minor lint fix

Signed-off-by: Timothy Chen <timothytim@google.com>
diff --git a/hw/ip/uart/rtl/uart_core.sv b/hw/ip/uart/rtl/uart_core.sv
index 11235ad..467fa3a 100644
--- a/hw/ip/uart/rtl/uart_core.sv
+++ b/hw/ip/uart/rtl/uart_core.sv
@@ -489,4 +489,8 @@
     .intr_o                 (intr_rx_parity_err_o)
   );
 
+  // unused registers
+  logic unused_reg;
+  assign unused_reg = ^reg2hw.alert_test;
+
 endmodule