staging: rtl8192u: Fixes symbol as static

This patch fixes "symbol 'ieee80211_send_probe_requests_rsl'
was not declared. Should it be static?" sparse warning
in ieee80211_softmac.c

Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>