staging: wlan-ng: fix bad symbol declaration

With sparse, the following error appears :
  CHECK   drivers/staging/wlan-ng/p80211netdev.c
drivers/staging/wlan-ng/cfg80211.c:710:6: warning: symbol 'prism2_connect_result' was not declared. Should it be static?
drivers/staging/wlan-ng/cfg80211.c:719:6: warning: symbol 'prism2_disconnected' was not declared. Should it be static?
drivers/staging/wlan-ng/cfg80211.c:725:6: warning: symbol 'prism2_roamed' was not declared. Should it be static?

Move functions declaration to coherent internal header file.

Signed-off-by: Neil 'Superna' Armstrong <superna9999@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>