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>