pwm: Add pwm_get_polarity() helper function

Some drivers are directly accessing the ->polarity field in pwm_device.
Add a helper to retrieve the current polarity so that we can easily move
this field elsewhere (required to support atomic update).

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