iwlwifi: mvm: disentangle paging command structs

Instead of using a union and hard-coding the size difference,
declare both command structs properly, use a union on the
stack to build them and the right sizeof() for the size.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>