pwm: sysfs: Make use of the DEVICE_ATTR_[RW][WO] macro's

For the npwm property the PWM sysfs interface already made use of the
DEVICE_ATTR_RO macro. This patch expands this to the other sysfs
properties so that the code base is concise and makes use of this
helpful macro.

This has the advantage of slightly reducing the code size, improving
readability and no longer using magic values for permissions.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>