spi: Fix types of the last chip select storage variables

spi: Fix types of the last chip select storage variables

First of all, last_cs_index_mask should be aligned with the original
cs_index_mask, which is 16-bit (for now) wide. Use the same pattern
for the last_cs_index_mask.

Second, last_cs can be negative and since 'char' is equal to 'unsigned
char' in the kernel, it's incorrect, strictly speaking, to assign
signed number to it. Use s8 type as it's done for *_native_cs ones.

With this change, regroup a bit the ordering to avoid too much memory
space to be wasted due to paddings. Shuffle kernel documentation
accordignly.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://msgid.link/r/20240307150256.3789138-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>