cfg80211: allow following country IE power for custom regdom cards

By definition WIPHY_FLAG_STRICT_REGULATORY was intended to allow the
wiphy to adjust itself to the country IE power information if the
card had no regulatory data but we had no way to tell cfg80211 that if
the card also had its own custom regulatory domain (these are typically
custom world regulatory domains) that we want to follow the country IE's
noted values for power for each channel. We add support for this and
document it.

This is not a critical fix but a performance optimization for cards
with custom regulatory domains that associate to an AP with sends
out country IEs with a higher EIRP than the one on the custom
regulatory domain. In practice the only driver affected right now
are the Atheros drivers as they are the only drivers using both
WIPHY_FLAG_STRICT_REGULATORY and WIPHY_FLAG_CUSTOM_REGULATORY --
used on cards that have an Atheros world regulatory domain. Cards
that have been programmed to follow a country specifically will not
follow the country IE power. So although not a stable fix distributions
should consider cherry picking this.

Cc: compat@orbit-lab.org
Cc: Paul Stewart <pstew@google.com>
Cc: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Cc: Senthilkumar Balasubramanian <senthilb@qca.qualcomm.com>
Reported-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>