pwm: sifive: Consistently name pwm_chip variables "chip"

Most variables holding a pointer to a pwm_chip are called "chip" which
is also the usual name in most other PWM drivers. Rename the single
variable that have a different name to be called "chip", too, for
consistency.

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