wifi: iwlwifi: mention the response structure in the kerneldoc

Add a comment to mention the structure used for the response for the
flush command.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Link: https://lore.kernel.org/r/20230127002430.422c9fbac12c.I2da0954d1c62007b5f01faf06df3e4081e52204f@changeid
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>