The sleep_auth value of the last interface to be set up prevailed when
an interface was removed. Take care of this by correctly configuring the
value according to the remaining STA/AP interfaces.

Take this opportunity to refactor the sleep_auth setting code for better
readability.

[Small style fix. -- Luca]
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>