mac80211: add supported rates change notification in IBSS

In IBSS it is possible that the supported rates set for a station changes over
time (e.g. it gets first initialised as an empty set because of no available
information about rates and updated later). In this case the driver has to be
notified about the change in order to update its internal table accordingly (if
needed).

This behaviour is needed by all those drivers that handle rc internally but
leave stations management to mac80211

Reported-by: Gui Iribarren <gui@altermundi.net>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
[Johannes - add docs, validate IBSS mode only, fix compilation]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>