pwm: iqs620a: Prepare removing pwm_chip from driver data

pwm: iqs620a: Prepare removing pwm_chip from driver data

This prepares the driver for further changes that will drop struct
pwm_chip chip from struct iqs620_pwm_private. The only two functions
that make use of struct iqs620_pwm_private::chip (apart from the probe
function that is handled later when converting to pwmchip_alloc()) only
use the chip's device pointer. Introduce such a pointer in driver data
and use that instead.

Link: https://lore.kernel.org/r/e6b1636c0d26d8bcb02a66b2963e2cc394014d76.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>