mac80211: allow mac80211 drivers to get to struct ieee80211_hw from wiphy

If a driver is given a wiphy and it wants to get to its private
mac80211 driver area it can use wiphy_to_ieee80211_hw() to get first
to its ieee80211_hw and then access the private structure via hw->priv. The
wiphy_priv() is already being used internally by mac80211 and drivers
should not use this. This can be helpful in a drivers reg_notifier().

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>