hostapd: Do not change HT40 capability due to OBSS scan

hostapd: Do not change HT40 capability due to OBSS scan

The capability itself isn't really affected by an OBSS
scan, only the HT operation must then be restricted to
20 MHz. Change this, and therefore use the secondary
channel configuration to determine the setting of the
OP_MODE_20MHZ_HT_STA_ASSOCED flag.

This shouldn't really change anything functionally,
it just makes the code a little less confusing and
is also needed to implement more dynamic bandwidth
changes if ever desired.

Signed-hostap: Johannes Berg <johannes.berg@intel.com>