pwm: core: Replace custom implementation of device_match_fwnode()

Replace custom implementation of the device_match_fwnode(). This hides
the implementation details and makes future changes easier.

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