spi: sprd: Convert to platform remove callback

Merge series from Uwe Kleine-König <u.kleine-koenig@pengutronix.de>:

An early error return from a remove callback is usally wrong. In the
case of the spi-sprd driver it's not that critical because the skipped
steps are mainly undoing the things that a successful runtime-resume
would have done.

Still it's cleaner to not exit early and not return an (mostly ignored)
error value. The second patch converts to .remove_new (which is the
motivation for this series).