pwm: img: Drop write-only variable from driver private data

struct img_pwm_chip::dev is only assigned to, but the member variable is
never used. So drop it.

Link: https://lore.kernel.org/r/7b7de753caa453c7fe591228c9a45d0baf8b5878.1707900770.git.u.kleine-koenig@pengutronix.de


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