Allow AP mode configuration with VHT enabled on 2.4 GHz

Allow AP mode configuration with VHT enabled on 2.4 GHz

hostapd_set_freq_param() rejected the 20 MHz channel case with
vht_enabled due to the existing validation step including only 5 GHz (to
be more exact, only >= 5000 MHz). While the behavior may not be fully
defined for 2.4 GHz, we can enable this based on driver capability
advertisement to fix automatic VHT selection for P2P use cases.
mac80211_hwsim advertises VHT for 2.4 GHz band and that resulted in
failures when trying to start GO on that band with vht=1 parameter.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>