rndis_wlan: cleanup: rename all rndis_wext* objects to rndis_wlan*

Driver used to be named rndis_wext before inclusion to upstream. Since
rndis_wlan is being converted to cfg80211, use of rndis_wext* names
can be confusing. So rename all rndis_wext to rndis_wlan (as should
have been when driver was renamed).
Signed-off-by: NJussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>