mac80211: simplify and clarify IE splitting

There's no need to split off IEs from the ones obtained
from userspace, if they were already split off, so for
example IEs that went before HT don't have to be listed
again to go before VHT. Simplify the code here so it's
clearer.

While at it, also clarify the comments regarding the DMG
(60 GHz) elements.

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