wl12xx: remove wext dependencies

This driver uses IW_ESSID_MAX_SIZE when it should
be using IEEE80211_MAX_SSID_LEN instead.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>