rndis_wlan: remove 'select WIRELESS_EXT' in Kconfig

Since rndis_wlan is now converted to cfg80211, WIRELESS_EXT isn't
required anymore.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>