serial: tegra: use NULL instead of 0

'cons' is a pointer; thus NULL should be used instead of 0.
Also, local symbols are staticized.
Fix the following sparse warnings:

drivers/tty/serial/serial-tegra.c:1209:27: warning: Using plain integer as NULL pointer
drivers/tty/serial/serial-tegra.c:1240:29: warning: symbol 'tegra20_uart_chip_data' was not declared. Should it be static?
drivers/tty/serial/serial-tegra.c:1246:29: warning: symbol 'tegra30_uart_chip_data' was not declared. Should it be static?
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial-tegra.c

@@ -1206,7 +1206,7 @@ static struct uart_driver tegra_uart_driver = {
 	.owner		= THIS_MODULE,
 	.driver_name	= "tegra_hsuart",
 	.dev_name	= "ttyTHS",
-	.cons		= 0,
+	.cons		= NULL,
 	.nr		= TEGRA_UART_MAXIMUM,
 };
 
@@ -1237,13 +1237,13 @@ static int tegra_uart_parse_dt(struct platform_device *pdev,
 	return 0;
 }
 
-struct tegra_uart_chip_data tegra20_uart_chip_data = {
+static struct tegra_uart_chip_data tegra20_uart_chip_data = {
 	.tx_fifo_full_status		= false,
 	.allow_txfifo_reset_fifo_mode	= true,
 	.support_clk_src_div		= false,
 };
 
-struct tegra_uart_chip_data tegra30_uart_chip_data = {
+static struct tegra_uart_chip_data tegra30_uart_chip_data = {
 	.tx_fifo_full_status		= true,
 	.allow_txfifo_reset_fifo_mode	= false,
 	.support_clk_src_div		= true,