pwm: img: Rename variable pointing to driver private data

Status quo is that variables of type struct img_pwm_chip * are named
"pwm_chip", "pwm" or "chip" which are all not optimal because there is a
struct pwm_chip in the core, usually only struct pwm_device * variables are
named "pwm" and "chip" is usually used for variabled of type struct
pwm_chip *.

So consistently use the same and non-conflicting name "imgchip".

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>