pwm: cros-ec: Simplify EC error handling

With enhanced error reporting from cros_ec_cmd_xfer_status() in place,
we can fully use it and no longer rely on EC error codes.

Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>