mac80211: update opmode when adding new station

Update the operating mode field is needed when an association
request contains the operating mode notification element and
it's not just changed later on the fly.

Signed-off-by: Marek Kwaczynski <marek.kwaczynski@tieto.com>
[clarify commit log, comments & fix whitespace]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>