serial: uartps: Fix missing unlock on error in cdns_get_id()

Add the missing unlock before return from function cdns_get_id()
in the error handling case.

Fixes: ae1cca3f ("serial: uartps: Change uart ID port allocation")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/xilinx_uartps.c

@@ -1394,8 +1394,10 @@ static int cdns_get_id(struct platform_device *pdev)
 	if (!alias_bitmap_initialized) {
 		ret = of_alias_get_alias_list(cdns_uart_of_match, "serial",
 					      alias_bitmap, MAX_UART_INSTANCES);
-		if (ret)
+		if (ret) {
+			mutex_unlock(&bitmap_lock);
 			return ret;
+		}
 
 		alias_bitmap_initialized++;
 	}