spi: Remove the now unused ctlr->idling flag

The ctlr->idling flag is never checked now, so we don't need to set it
either.

Signed-off-by: David Jander <david@protonic.nl>
Link: https://lore.kernel.org/r/20220621061234.3626638-8-david@protonic.nl


Signed-off-by: Mark Brown <broonie@kernel.org>