carl9170: configurable beacon rates

Previously, the beacon rate was fixed to either:
 * 1Mb/s [2.4GHz band]
 * 6Mb/s [5GHz band]

This limitation has been addressed and now the
beacon rate is selected by ieee80211_tx_info's
rate control info, almost like any ordinary
data frame.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>