spi: document CS setup, hold & inactive times in header

This change documents the CS setup, host & inactive times. They were
omitted when the fields were added, and were caught by one of the build
bots.

Fixes: 25093bde ("spi: implement SW control for CS times")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20191023070046.12478-1-alexandru.ardelean@analog.com


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