wifi: cfg80211: annotate iftype_data pointer with sparse

There were are a number of cases in mac80211 and iwlwifi (at
least) that used the sband->iftype_data pointer directly,
instead of using the accessors to find the right array entry
to use.

Make sparse warn when such a thing is done.

To not have a lot of casts, add two helper functions/macros

 - ieee80211_set_sband_iftype_data()
 - for_each_sband_iftype_data()

Signed-off-by: Johannes Berg <johannes.berg@intel.com>