ath10k: dump tx stats in rate table format

This patch adds the tx statistics pkts/bytes dump
in rate table format.

Dump format table is similar to http://mcsindex.com/



Tested on QCA9984/QCA4019/QCA988x
Firmware: 10.4-3.5.3-00057
	  10.2.4-1.0-00037
command:
cat /sys/kernel/debug/ieee80211/phy0/netdev\:wlan0/
    stations/<MACADDR>/tx_stats

Signed-off-by: Anilkumar Kolli <akolli@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>