pwm: sunxi: Use of_device_get_match_data()

The usage of of_device_get_match_data reduce the code size a bit.
Furthermore, it prevents an improbable dereference when
of_match_device() returns NULL.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>