ath10k: add bitrate parse for peer stats info

The rate code and rate kbps report by WMI_TLV_PEER_STATS_INFO_EVENTID
from firmware contains all the bitrate info which include OFDM, CCK,
HT/VHT, and mac80211 need the struct rate_info which include below
parameters:

flags: bitflag of flags from &enum rate_info_flags
mcs: mcs index if struct describes an HT/VHT/HE rate
legacy: bitrate in 100kbit/s for 802.11abg
nss: number of streams (VHT & HE only)
bw: bandwidth (from &enum rate_info_bw)

For OFDM/CCK, its rate kbps indicate the bitrate, for HT/VHT, mac80211
need the above 5 parameters to cacluate the bitrate and show by iw.

After parse the bitrate info, iw show the correct rx bitrate:

localhost ~ # iw wlan0 link
	rx bitrate: 234.0 MBit/s VHT-MCS 3 80MHz VHT-NSS 2
	rx bitrate: 40.5 MBit/s MCS 2 40MHz
	rx bitrate: 72.2 MBit/s MCS 7 short GI
	rx bitrate: 54.0 MBit/s
	rx bitrate: 48.0 MBit/s

Tested with QCA6174 SDIO with firmware WLAN.RMH.4.4.1-00042.

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200427080416.8265-4-wgong@codeaurora.org