pwm: Use the DT macro directly when parsing PWM DT flags

Don't redefine a PWM_SPEC_POLARITY macro with a value identical to
PWM_POLARITY_INVERTED, use the PWM DT macro directly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>