rt2x00: change default MAC_BSSID_DW1_BSS_BCN_NUM

We setup MAC_BSSID_DW1_BSS_BCN_NUM dynamically when numbers of active
beacons increase. Change default to 0 to tell hardware that we want to
send only one beacon as default.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>