iwlagn: fill beacon TX rate properly

Use the rate that mac80211 requested to fill the
uCode TX command for the beacon. Unfortunately,
the uCode is (currently?) ignoring it, but now
at least fill it properly.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>