hostapd: Fix length of Bandwidth Indication subelement
The default length of the Bandwidth Indication subelement should be
equal to the minimum size of ieee80211_bw_ind_element structure. The
previously used value truncated this subelement by one octet.
Fixes: c7e704bdf9c3 ("hostapd: Add Bandwidth Indication subelement support for channel switch") Signed-off-by: Shayne Chen <shayne.chen@mediatek.com> Signed-off-by: Allen Ye <allen.ye@mediatek.com>