pwm: ab8500: Add error message if pwmchip_add() fails

pwmchip_add() doesn't emit an error message, so add one in the driver.

Signed-off-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>