leds: pwm: Make error handling more robust

It's easy to miss necessary clean up, e.g. firmware node reference counting,
during error path in ->probe(). Make it more robust by moving to a single
point of return.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>