pwm: sl28cpld: Consistently name pwm_chip variables "chip"

Nearly all PWM drivers use the name "chip" for the member in the driver
struct pointing to the pwm_chip. Also all local variables and function
parameters with this type use this name. Rename the struct pwm_chip
member accordingly for consistency.

Also rename the parameter of the macro sl28cpld_pwm_from_chip to "chip".

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