ath10k: set a-mpdu receiver reference number

Set the a-mpdu reference number in ath10k to make it accessible in the
receivers radiotap header. Implemented as in ath9k.  The reference number is
needed for troubleshooting and research at the receivers site (e.g. to identify
mpdu's that were aggregated in an a-mpdu)

Signed-off-by: Matthias Frei <mf@frei.media>
[kvalo@qca.qualcomm.com: fix checkpatch warning, commit log cleanup]
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>