spi: update the structure documentation

some members were not described in documentation.

Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Link: https://lore.kernel.org/r/1583774179-30736-1-git-send-email-hqjagain@gmail.com


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