pwm: tegra: Use NSEC_PER_SEC

Instead of using the literal value for the number of nanoseconds per
second, use the macro instead to increase readability.
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-tegra.c

@@ -87,7 +87,7 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	 * cycles at the PWM clock rate will take period_ns nanoseconds.
 	 */
 	rate = clk_get_rate(pc->clk) >> PWM_DUTY_WIDTH;
-	hz = 1000000000ul / period_ns;
+	hz = NSEC_PER_SEC / period_ns;
 
 	rate = (rate + (hz / 2)) / hz;