Prioritize EDCA by choosing the AC before anything else. Use the
fast/slow link bitmap in FW to improve the scheduling algorithm for
the multi-link scenario.

Set packet thresholds to determine if a given link is high or low
priority according to its speed. A slow link will be given high priority
if the amount of packets queued for it in the FW is lower than the
slow-threshold. Similarly, a fast link will be given high priority if
the number of its packets queued in FW is smaller than the high-threshold.

The improved algorithm:
1. Choose optimal AC according to FW utilization
2. Traversing the VIFs in a round-robin fashion, try to choose a high
   priority link. Links are traversed in a round-robin fashion inside a
   VIF.
3. If no high priority links are found, choose the first non-empty
   (low priority) link found in the round robin.
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>