pwm: Constify PWM device where possible

The PWM argument is not modified in PWM property accessors, make it a
const argument so that the accessors can be used from sysfs.
Signed-off-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>

include/linux/pwm.h

@@ -103,7 +103,7 @@ static inline void pwm_set_period(struct pwm_device *pwm, unsigned int period)
 		pwm->period = period;
 }
 
-static inline unsigned int pwm_get_period(struct pwm_device *pwm)
+static inline unsigned int pwm_get_period(const struct pwm_device *pwm)
 {
 	return pwm ? pwm->period : 0;
 }
@@ -114,7 +114,7 @@ static inline void pwm_set_duty_cycle(struct pwm_device *pwm, unsigned int duty)
 		pwm->duty_cycle = duty;
 }
 
-static inline unsigned int pwm_get_duty_cycle(struct pwm_device *pwm)
+static inline unsigned int pwm_get_duty_cycle(const struct pwm_device *pwm)
 {
 	return pwm ? pwm->duty_cycle : 0;
 }