pwm: sun4i: Rename variable pointing to driver private data

Status quo is that variables of type struct sun4i_pwm_chip * are named
"pwm". This name is usually reserved for variabled of type struct
pwm_chip *.

So consistently use the same and non-conflicting name "sun4ichip" which
better reflects the intend

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