drivers: eth: gmac: fix qav bandwidth sharing issues
drivers: eth: gmac: fix qav bandwidth sharing issues

Removing the TX timeout handling in the GMAC driver (commit 18b07e09e0)
revealed some issues with the way hardware priority queues work.

For cases with both hardware priority queues enabled, with the default
recommended delta bandwidths (0% - 75%), the lower priority queue (0%)
is hardly able to send any packets. This became visible, because without
the timeout mechanism, we quickly ran out of available TX buffers if
there were multiple packets being queued to the queue.

Here is an excerpt from 802.1Q, chapter 34.3.1 which describes how Qav
bandwidth sharing SHOULD work:

  The deltaBandwidth(N) for a given N, plus the deltaBandwidth(N) values
  for any higher priority queues (larger values of N) defines the total
  percentage of the Port’s bandwidth that can be reserved for that queue
  and all higher priority queues. For the highest priority queue, this
  means that the maximum value of operIdleSlope(N) is deltaBandwidth(N)%
  of portTransmitRate. However, if operIdleSlope(N) is actually less
  than this maximum value, any lower priority queue that supports the
  credit-based shaper algorithm can make use of the reservable bandwidth
  that is unused by the higher priority queue. So, for queue N-1, the
  maximum value of (operIdleSlope(N) + operIdleSlope(N-1)) is
  (deltaBandwidth(N) + deltaBandwidth(N1))% of portTransmitRate.

However in reality, the lower priority queues (N-1) on the SAM GMAC
hardware DO NOT use the bandwidth available from the higher priority
queues (N).

This commits fixes the issue by changing the defaults. These are still
set to the recommended 75% (total), but this percentage is split between
the priority queues manually.

Signed-off-by: Tomasz Gorochowik <265c7c25eca9aa9127910457b0aea7e793d329f1@antmicro.com>

Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,ldts/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr