mac80211: never pass NULL params to ieee80211_if_add()

This isn't really a problem now, but it means that the function
has a few NULL checks that are only relevant when coming from
the initial interface added in mac80211, and that's confusing.
Just pass non-NULL (but equivalently empty) in that case and
remove all the NULL checks.

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