pwm: Don't initialize list head before calling list_add()

list_add() just overwrites the members of the element to add (here:
chip->list) without any checks, even in the DEBUG_LIST case. So save the
effort to initialize the list.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20221117211143.3817381-5-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>