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>

drivers/pwm/pwm-ab8500.c

@@ -101,7 +101,7 @@ static int ab8500_pwm_probe(struct platform_device *pdev)
 
 	ab8500->chip.dev = &pdev->dev;
 	ab8500->chip.ops = &ab8500_pwm_ops;
-	ab8500->chip.base = pdev->id;
+	ab8500->chip.base = -1;
 	ab8500->chip.npwm = 1;
 
 	err = pwmchip_add(&ab8500->chip);