iwlwifi: mvm: split ADD_STA and ADD_STA_KEY in firmware API

Add support for new station management firmware API. The old
ADD_MODIFY_STA command has been replaced with two: a modified
ADD_MODIFY_STA and a new ADD_MODIFY_STA_KEY command.

Signed-off-by: Max Stepanov <Max.Stepanov@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>