cfg80211: clean up naming once and for all

We've named the registered devices 'drv' sometimes,
thinking of "driver", which is not what it is, it's
the internal representation of a wiphy, i.e. a
device. Let's clean up the naming once and and use
'rdev' aka 'registered device' everywhere.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>