wil6210: add TX latency statistics

Collect statistics of TX latency. The latency is measured from the time
the HW gets aware of new SKB to transmit until the HW indicates tx
complete for this SKB.
The statistics are shown via new "tx_latency" debugfs.

Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Git-commit: a24a3d6abb978d4abc25d541e787981e7ef555c8
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git


Change-Id: I0bd10d4b1a0b21eeded119bf5cf52e6c16e88515
Signed-off-by: Maya Erez <merez@codeaurora.org>