rt2x00: Remove usage of IEEE80211_CONF_CHANGE_BEACON_INTERVAL

IEEE80211_CONF_CHANGE_BEACON_INTERVAL was deprecated a month ago,
it is about time to remove all usage from the rt2x00 drivers and
use the correct beacon interval configuration through the bss_info
structure.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>