pwm: Add the pwm_is_enabled() helper

Some PWM drivers are testing the PWMF_ENABLED flag. Create a helper
function to hide the logic behind enabled test. This will allow us to
smoothly move from the current approach to an atomic PWM update
approach.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
diff --git a/drivers/pwm/pwm-renesas-tpu.c b/drivers/pwm/pwm-renesas-tpu.c
index ee63f9e..075c1a76 100644
--- a/drivers/pwm/pwm-renesas-tpu.c
+++ b/drivers/pwm/pwm-renesas-tpu.c
@@ -301,7 +301,7 @@
 	pwm->duty = duty;
 
 	/* If the channel is disabled we're done. */
-	if (!test_bit(PWMF_ENABLED, &_pwm->flags))
+	if (!pwm_is_enabled(_pwm))
 		return 0;
 
 	if (duty_only && pwm->timer_on) {