wireless: mark cfg80211_is_all_idle as static

  CHECK   net/wireless/sme.c
net/wireless/sme.c:38:6: warning: symbol 'cfg80211_is_all_idle' was not declared. Should it be static?

It is not used elsewhere, so mark it static.

Signed-off-by: John W. Linville <linville@tuxdriver.com>