mac80211: fix the for_each_sta_info macro

Because of an ambiguity in the for_each_sta_info macro, it can
currently only be used if the third parameter is set to 'sta'.
Fix this by renaming the parameter to '_sta'.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>