mac80211: free management frame keys when removing station

When writing the code to allow per-station GTKs, I neglected to
take into account the management frame keys (index 4 and 5) when
freeing the station and only added code to free the first four
data frame keys.

Fix this by iterating the array of keys over the right length.

Cc: stable@vger.kernel.org
Fixes: e31b8213 ("cfg80211/mac80211: allow per-station GTKs")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>