mac80211: fix warning: unused variable ifsta

This patch fixes warning unused variable ifsta
when compiling without CONFIG_MAC80211_VERBOSE_DEBUG

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>