mac80211: adjust initial chandefs assignments in ieee80211_register_hw

I noticed that monitor interfaces by default would start on 5GHz
while STA/AP ones would start 2GHZ - It stems from the fact that
ieee80211_register_hw unnecessarily adjusts the local->monitor_chandef
for each band.

This avoids this and while at it uses a single dflt_chandef to initialize
in one go local->{hw.conf.chandef,_oper_chandef,monitor_chandef}

Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>