staging: rtl8192u: Fix static decleration sparse warning.

The function 'ieee80211_check_auth_response' is used only in this
file, so make it static. This patch fixes the following sparse
warning.

'ieee80211_check_auth_response' was not declared. Should it be static?

Signed-off-by: Cihangir Akturk <cakturk@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>