staging: wfx: fix QoS priority for slow buses

The device is in charge of respecting the QoS constraints. The driver
have to ensure that all the queues contain data and the device choose
the right queue to send.

The things starts to be more difficult when the bandwidth of the bus is
lower than the bandwidth of the WiFi. The device quickly sends the
frames of the highest priority queue. Then, it starts to send frames
from a lower priority queue. Though, there are still some high priority
frames waiting in the driver.

To work around this problem, this patch add some priorities to each
queue. The weigh of the queue was (roughly) calculated experimentally by
checking the speed ratio of each queue when the bus does not limit the
traffic:
    - Be/Bk -> 20Mbps/10Mbps
    - Vi/Be -> 36Mbps/180Kbps
    - Vo/Be -> 35Mbps/600Kbps
    - Vi/Vo -> 24Mbps/12Mbps

So, if we fix the weigh of the Background to 1, the weight of Best
Effort should be 2. The weight of Video should be 116. However, since
there is only 32 queues, it make no sense to use a value greater than
64[1]. And finally, the weight of the Voice is set to 128.

[1] Because of this approximation, with very slow bus, we can still
observe frame starvation when we measure the speed ratio of Vi/Be. It is
around 35Mbps/1Mbps (instead of 36Mbps/180Kbps). However, it is still in
accepted error range.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20201007101943.749898-5-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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