soc/nvidia/tegra210: Add null pointer check

Check that tx is not null before accessing it, similar to the previous
if statements.

Change-Id: I820cb670026bb12a54c63227aa04e778fd49c66a
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Found-by: Coverity CID 1294805
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34530
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/soc/nvidia/tegra210/dsi.c

@@ -769,7 +769,7 @@ static ssize_t tegra_dsi_host_transfer(struct mipi_dsi_host *host,
 	tegra_dsi_writel(dsi, value, DSI_WR_DATA);
 
 	/* write payload (if any) */
-	if (msg->tx_len > 2) {
+	if (tx && msg->tx_len > 2) {
 		for (j = 2; j < msg->tx_len; j += 4) {
 			value = 0;