cfg80211: change registered device pointer name

Name "dev" is too common and ambiguous, let all the pointer name
pointing to struct cfg80211_registered_device be "rdev". This can
improve code readability and consistency(since other places have
already called it rdev).

Signed-off-by: Zhao, Gang <gamerh2o@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>