rt2x00: Remove last usage of beacon_int from ieee80211_config

This removes the last usage of beacon_int inside the iee80211_config
structure from rt2x00. The attempt is a bit hackish, and subject to
change in the future when the entire rt2x00_dev structure is
cleaned up and restructured.

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