spi: Consistently use BIT for cs_index_mask (part 2)

For some reason the commit 1209c556 ("spi: Consistently use BIT
for cs_index_mask") missed one place to change, do it here to finish
the job.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240415184757.1198149-1-andriy.shevchenko@linux.intel.com


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