iwlwifi: remove command callback return value

No existing callbacks use anything other than the return
value 1, which means that the caller should free the
reply skb, so it seems safer in terms of not introducing
memory leaks to simply remove the return value and let
the caller always free the skb.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>