rndis_wlan: do link-down state change in worker thread

rndis_wext_link_change() is called from within rndis_command() so it
isn't very good place to do any work. Move to worker thread.

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