libertas: release bss references and avoid warning from cfg80211_inform_bss

  CC [M]  drivers/net/wireless/libertas/cfg.o
drivers/net/wireless/libertas/cfg.c: In function ‘lbs_ret_scan’:
drivers/net/wireless/libertas/cfg.c:636:24: warning: ignoring return value of ‘cfg80211_inform_bss’, declared with attribute warn_unused_result
drivers/net/wireless/libertas/cfg.c: In function ‘lbs_join_post’:
drivers/net/wireless/libertas/cfg.c:1766:21: warning: ignoring return value of ‘cfg80211_inform_bss’, declared with attribute warn_unused_result

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