spi: axi-spi-engine: fix struct member doc warnings

spi: axi-spi-engine: fix struct member doc warnings

The build bots are complaining that the members of struct
spi_engine_message_state are not described. This adds the
proper @name: syntax to the comments to fix this.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312182101.QOWovo29-lkp@intel.com/
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://msgid.link/r/20231218145348.339470-1-dlechner@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>