pwm: rcar: Document inability to set duty_cycle = 0

When .apply() is called with state->duty_cycle = 0 the duty_ns parameter
to rcar_pwm_set_counter() is 0 which results in ph being 0 and
rcar_pwm_set_counter() returning -EINVAL.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-rcar.c

@@ -3,6 +3,9 @@
  * R-Car PWM Timer driver
  *
  * Copyright (C) 2015 Renesas Electronics Corporation
+ *
+ * Limitations:
+ * - The hardware cannot generate a 0% duty cycle.
  */
 
 #include <linux/clk.h>