spi: Exctract spi_dev_check_cs() helper

It seems a few functions implement the similar for-loop to validate
chip select pins for uniqueness. 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/20240306160114.3471398-3-andriy.shevchenko@linux.intel.com


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