spi: make remove callback a void function

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

this series goal is to change the spi remove callback's return value to void.
After numerous patches nearly all drivers already return 0 unconditionally.
The four first patches in this series convert the remaining three drivers to
return 0, the final patch changes the remove prototype and converts all
implementers.

base-commit: 26291c54