rt2x00: change beaconing setup on RT2800

As reported by Matthias, on 5572 chip, even if we clear up TXWI
of corresponding beacon, hardware still try to send it or do other
action that increase power consumption peak up to 1A.

To avoid the issue, setup beaconing dynamically by configuring offsets
of currently active beacons and MAC_BSSID_DW1_BSS_BCN_NUM variable,
which limit number of beacons that hardware will try to send.

Reported-by: Matthias Fend <Matthias.Fend@wolfvision.net>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>