mac80211: use RCU read locks for sta_info_get

this is being recently introduced by the commit
a85e1d55



Cc: Paul Stewart <pstew@google.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>