mac80211: Update last_tx_rate only for data frames

The last_tx_rate field was also updated for non-data frames that are
often sent with a lower rate (for example management frames at 1 Mbps).
This is confusing when the data rate is actually much higher.

Hence, only update the last_tx_rate field with tx rate information
gathered from last data frames.

If the rate control algorithm filled in txrc.reported_rate we don't need
to verify this information.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>