Rate controller in firmware may also return the Tx Rate
used for management frame that is usually sent as lowest
Tx Rate (1Mbps in 2.4GHz). So update the last_tx_rate only
if it is data frame.

This patch is tested with ath9k_htc.
Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>