cfg80211: constify more pointers in the cfg80211 API

cfg80211: constify more pointers in the cfg80211 API

This also propagates through the drivers.

The orinoco driver uses the cfg80211 API structs for internal
bookkeeping, and so needs a (void *) cast that removes the
const - but that's OK because it allocates those pointers.

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