pwm: lpc18xx-sct: Drop hardly used member from driver private data

The device pointer is only used once in lpc18xx_pwm_request(). There the
pwm_chip is available, too, which has a pointer to the same structure.
Replace the usage accordingly and drop the then write-only member
variable.

Link: https://lore.kernel.org/r/427a719a24dd7ff5ebbac3e260a1773c094c7489.1707900770.git.u.kleine-koenig@pengutronix.de


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