wl12xx: simplify wl1271_ssid_set()

Simplify wl1271_ssid_set by re-using cfg80211_find_ie instead of
reimplementing it.

Additionally, add a length check to prevent a potential buffer overflow.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>