pwm: devm: alloc correct pointer size

The allocated object should be the size of what the pointer is pointing
to and not the size of the pointer itself.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>