pwm: ab8500: Fix register offset calculation to not depend on probe order

The assumption that lead to commit 5e5da1e9 ("pwm: ab8500:
Explicitly allocate pwm chip base dynamically") was wrong: The
pwm-ab8500 devices are not directly instantiated from device tree, but
from the ab8500 mfd driver. So the pdev->id isn't -1, but a number
between 1 and 3. Now that pwmchip ids are always allocated dynamically,
this cannot easily be reverted.

Introduce a new member in the driver data struct that tracks the
hardware id and use this to calculate the register offset.

Side-note: Using chip->base to calculate the offset was never robust
because if there was already a PWM with id 1 at the time ab8500-pwm.1
was probed, the associated pwmchip would get assigned chip->base = 2 (or
something bigger).

Fixes: 5e5da1e9 ("pwm: ab8500: Explicitly allocate pwm chip base dynamically")
Fixes: 6173f8f4

 ("pwm: Move AB8500 PWM driver to PWM framework")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>