wm97xx_remove() returns zero unconditionally. To prepare changing the
prototype for platform remove callbacks to return void, make it explicit
that wm97xx_mfd_remove() always returns zero.

The prototype for wm97xx_remove cannot be changed, as it's also used as
a plain device remove callback.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20220708062718.240013-1-u.kleine-koenig@pengutronix.deSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>