spi: Remove unneeded READ_ONCE for ctlr->busy flag

Now this flag is written entirely in the mutex, so no need for READ_ONCE

Signed-off-by: David Jander <david@protonic.nl>
Link: https://lore.kernel.org/r/20220621061234.3626638-9-david@protonic.nl


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