regulatory: clarify locking rules and assertions

Many places that currently check that cfg80211_mutex
is held don't actually use any data protected by it.
The functions that need to hold the cfg80211_mutex
are the ones using the cfg80211_regdomain variable,
so add the lock assertion to those and clarify this
in the comments.

The reason for this is that nl80211 uses the regdom
without being able to hold reg_mutex.

Acked-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>