hostapd: make ACS dependent on nl80211

hostapd: make ACS dependent on nl80211

According to hostapd's defconfig file, ACS is currently only supported
through the nl80211 driver. Search through the source code also confirms
that ACS machinery is present in the nl80211 driver only.

Although hostapd can be built with ACS enabled and nl80211 disabled, an
attempt to use ACS with other drivers by setting a wireless channel to 0
results in a runtime failure (driver doesn't accept this value). So we
might save a user from selecting a meaningless combination by making ACS
dependent on nl80211.

Signed-off-by: Alexander Mukhin <alexander.i.mukhin@gmail.com>
Reviewed-by: Sam Voss <sam.voss@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>