hostapd: add a require_mode option in wifi-device sections to select the minimum hardware mode that the AP requires from clients

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41665 3c298f89-4303-0410-b956-a3cf2f4a3e73