linux-phytec-fsl

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: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>