mac80211: update last_tx_rate only for data frame

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>