nl80211: Add basic rate configuration for AP mode

Add a new attribute, NL80211_ATTR_BSS_BASIC_RATES, that can be used with
NL80211_CMD_SET_BSS for userspace (e.g., hostapd) to set which rates are
in the basic rate set.

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>