spi: Exctract spi_set_all_cs_unused() helper

It seems a few functions implement the similar for-loop to mark all
chip select pins unused. Let's deduplicate that code in order to have
a single place of that for better maintenance.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://msgid.link/r/20240306160114.3471398-2-andriy.shevchenko@linux.intel.com


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