pwm: omap-dmtimer: Prepare removing pwm_chip from driver data

This prepares the driver for further changes that will drop struct
pwm_chip chip from struct pwm_omap_dmtimer_chip. Use the pwm_chip as
driver data instead of the pwm_omap_dmtimer_chip to get access to the
pwm_chip in pwm_omap_dmtimer_remove() without using omap->chip.

Link: https://lore.kernel.org/r/809672026f3d583ba4fc4bd92b525951df172106.1707900770.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>