Clean up hostapd_get_he_twt_responder() processing

Clean up hostapd_get_he_twt_responder() processing

mode->he_capab is an array and as such, there is no point in checking
whether it is NULL since that cannot be the case. Check for the
he_supported flag instead. In addition, convert the TWT responder
capability bit into a fixed value 1 to avoid any surprising to the
callers. In practice, neither of these changes results in different
behavior in the current implementation, but this is more robust.

Signed-off-by: Jouni Malinen <j@w1.fi>