mac80211: fix disabling of 802.11n features

r40682 ("mac80211: clean up ht capability handling, drop the use of the
ht_capab list, use individual variables instead") removed the ht_capab list and
replaced it with optional variables to disable features for a phy. But these
variables weren't added in drv_mac80211_init_device_config and thus didn't make
any difference when modifying /etc/config/wireless.

Signed-off-by: Sven Eckelmann <sven@narfation.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41180 3c298f89-4303-0410-b956-a3cf2f4a3e73