pwm: pwm-mediatek: Simplify error handling with dev_err_probe()

Use dev_err_probe() to simplify handling errors in pwm_mediatek_probe().

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>