pwm: core: Reuse fwnode_to_pwmchip() in ACPI case

In ACPI case we may use matching by fwnode as provided via
fwnode_to_pwmchip(). This makes device_to_pwmchip() not needed
anymore.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>