mac80211: extend ieee80211_ie_split to support EXTENSION

Current ieee80211_ie_split() implementation doesn't
account for elements that are sub-elements of the
EXTENSION IE. To extend support to these IEs as well,
treat the WLAN_EID_EXTENSION ids in the %ids array
as indicating that the next id in the array is a
sub-element of the EXTENSION IE.

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