revert "mwifiex: enable aggregation for TID 6 and 7 streams"

This patch reverts 'commit c5534844 ("mwifiex:
enable aggregation for TID 6 and 7 streams")'.
TID6 & TID7 is voice traffic which is quite time sensitive.
Do not enable any type of aggregation on these TIDs.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>