Staging: wlan-ng: fix sparse warnings for static functions

Fix sparse warnings for static functions
* drivers/staging/wlan-ng/cfg80211.c:101:5: warning:
symbol 'prism2_change_virtual_intf' was not declared.

Signed-off-by: Teodora Baluta <teobaluta@gmail.com>
Signed-off-by: Zach Brown <zab@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>