pwm: ab8500: Fix error code in probe()

This code accidentally return positive EINVAL instead of negative
-EINVAL.

Fixes: eb41f334

 ("pwm: ab8500: Fix register offset calculation to not depend on probe order")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>