hostapd: SET ht_capab support for disabling 40 MHz bandwidth

hostapd: SET ht_capab support for disabling 40 MHz bandwidth

'hostapd_cli SET ht_capab' only checked for [HT40+] or [HT40-] or both
to be present. Based on the offset + or -, secondary_channel is updated
but HT20/VHT20 mode can be brought up only from config file and can't be
done using the SET command when the current HT mode is HT40+ or HT40-.

When managing AP+STA mode from userspace doing hostapd_cli: "disable ->
set channel, ht_capab -> enable" sequence, channel switch from
HT40/VHT40 to HT20/VHT20 was not possible with this SET ht_capab
limitation.

Cover this additional case by resetting secondary_channel to 0 for
HT20/VHT20 when ht_capab has neither [HT40+] nor [HT40-] present.

Signed-off-by: Sathishkumar Muruganandam <murugana@codeaurora.org>