spi: Remove check for controller idling in spi sync path

Now that the idling flag is wholly behind the io_mutex, this broken piece
of code can be safely removed.

Signed-off-by: David Jander <david@protonic.nl>
Link: https://lore.kernel.org/r/20220621061234.3626638-6-david@protonic.nl


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