staging: greybus: pwm: Consistently name pwm_chip variables "chip"

All function parameters of type pointer to struct pwm_chip in this
driver are called chip which is also the usual name of function
parameters and local variables in most other PWM drivers. For
consistency use the same name for the local variable of that type.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>