drm/tegra: dsi: Replace 1000000000UL by NSEC_PER_SEC

Using the symbolic constant instantly provides a lot more context.
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/dsi.c

@@ -349,7 +349,7 @@ static int tegra_dsi_set_phy_timing(struct tegra_dsi *dsi)
 	if (rate < 0)
 		return rate;
 
-	period = DIV_ROUND_CLOSEST(1000000000UL, rate * 2);
+	period = DIV_ROUND_CLOSEST(NSEC_PER_SEC, rate * 2);
 
 	err = mipi_dphy_timing_get_default(&timing, period);
 	if (err < 0)