sdhci_pltfm_unregister() returns 0 unconditionally and returning an
error in a platform remove callback isn't very sensible. (The only
effect of the latter is that the device core emits a generic warning and
then removes the device anyhow.)

So return 0 unconditionally to make it obvious there is no error
forwarded to the upper layers.

This is a preparation for making platform remove callbacks return void.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20220610211257.102071-4-u.kleine-koenig@pengutronix.deSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>