pwm: xilinx: Prepare removing pwm_chip from driver data

pwm: xilinx: Prepare removing pwm_chip from driver data

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

Link: https://lore.kernel.org/r/738b9929c1d13bde64050f8bbc4ce8d85f58cc7a.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>