nl80211: don't put struct cfg80211_ap_settings on stack

This struct has grown quite a bit, so dynamically allocate
it instead of putting it on the stack.

Link: https://lore.kernel.org/r/20210923161836.5813d881eae3.I0fc0f83905b0bfa332c4f1505e00c13abfca3545@changeid


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