mac80211: Fix sparse warning for ssid_len on ieee80211_sta_config_auth()

net/mac80211/mlme.c:2079:28: warning: symbol 'ssid_len' shadows an earlier one
net/mac80211/mlme.c:2022:12: originally declared here

ssid_len is already being declared and checked above so there is
no need for it again.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>