spi: Lock controller idling transition inside the io_mutex

This way, the spi sync path does not need to deal with the idling
transition.

Signed-off-by: David Jander <david@protonic.nl>
Link: https://lore.kernel.org/r/20220621061234.3626638-4-david@protonic.nl


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