ath11k: Add dp tx err stats

Add support for dp tx error stats which logs tx failure reasons due
to ring full condition, etc. This stats is added in soc_dp_stats
which was earlier used as soc_rx_stats so that all dp related info
are logged in same file.

Below is an example usage,

root@OpenWrt:/# cat /sys/kernel/debug/ath11k/ipq8074/soc_dp_stats
SOC RX STATS:

err ring pkts: 0
Invalid RBM: 0
<snip>

SOC TX STATS:

Ring Full Failures:
ring0: 4
ring1: 3
ring2: 5

Misc Transmit Failures: 2

Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.1.0.1-01213-QCAHKSWPL_SILICONZ-1

Signed-off-by: Sriram R <srirrama@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1591768308-32005-2-git-send-email-srirrama@codeaurora.org