hostapd: make ACS optional

Make ACS (Automatic Channel Selection) support in hostapd optional.
There's a clash of standard vs. propietary solutions, hence when hostapd
has builtin ACS support (standard) and it's enabled (channel=0 in the
config) this will preclude the propietary driver from using ACS (it
won't work at all).

See:
http://e2e.ti.com/support/wireless_connectivity/f/307/p/465333/1669270



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>