wl12xx: set authorized AP on sta_state notification

wl12xx currently looks for AP authorization by registering
a netdev notifier and waiting for the IF_OPER_UP notification,
which is quite cumbersome.

Use the newly introduced sta_state callback (waiting
for assoc -> auth notification) instead, in order to
simplify it.

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