pwm: ab8500: Explicitly allocate pwm chip base dynamically

The ab8500 driver is the last one which doesn't (explicitly) use dynamic
allocation of the pwm id. Looking through the kernel sources I didn't
find a place that relies on this id. And with the device probed from
device tree pdev->id is -1 anyhow; making this explicit looks
beneficial, too.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>