pwm: core: Unify fwnode checks in the module

Historically we have two different approaches on how to check type of fwnode.
Unify them using the latest and greatest fwnode related APIs.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>