hostapd: Add uapsd option to netifd.sh

The uapsd option sets the uapsd_advertisement_enabled flag in hostapd.

The check for phy support is already implemented here in hostapd since 2011:
http://w1.fi/cgit/hostap/commit/?id=70619a5d8a3d32faa43d66bcb1b670cacf0c243e



So this can be safely set to 1 as default.

Signed-off-by: Vittorio Gambaletta <openwrt@vittgam.net>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43846 3c298f89-4303-0410-b956-a3cf2f4a3e73