mac80211/cfg80211: Add BSS configuration options for AP mode

This change adds a new cfg80211 command, NL80211_CMD_SET_BSS, to allow
AP mode BSS parameters to be changed from user space (e.g., hostapd).
The drivers using mac80211 are expected to be modified with separate
changes to use the new BSS info parameter for short slot time in the
bss_info_changed() handler.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>