mac80211: don't expose ieee80211_add_srates_ie()

This and ieee80211_add_ext_srates_ie() aren't
exported, so can't be used by drivers anyway,
but there's also no reason that they should be
so make them private to mac80211 and use sdata
instead of vif arguments.

Acked-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>