ap: Fix invalid HT40 channel pair fallback

ap: Fix invalid HT40 channel pair fallback

In case of incorrect HT40 configuration as part of an attempt to create
a 80 MHz AP, iface->conf->vht_oper_centr_freq_seg0_idx and
iface->conf->vht_oper_centr_freq_seg1_idx are zero'ed, but
iface->conf->vht_oper_chwidth remains VHT_CHANWIDTH_80MHZ. This causes
the logic in dfs_get_start_chan_idx to fail.

Fix this by setting iface->conf->vht_oper_chwidth to
VHT_CHANWIDTH_USE_HT when zero'ing the center frequency parameters.

Signed-off-by: Naftali Goldstein <naftali.goldstein@intel.com>