ath10k: Fix mpdu spacing to 8us

The FW doesn't fill the mpdu density field in the HT
capabilities received in the service_ready event and
the host driver ends up advertising 0 in its capabilities.

Fix this by hard-coding the spacing time to 8us.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>