net: bcmgenet: Use weighted round-robin TX DMA arbitration

Under heavy network traffic, we observed sporadic TX queue timeouts on the
Raspberry Pi 4. The timeouts can be reproduced by stress testing the TX
path with multiple concurrent iperf UDP streams:

    iperf3 -c <ip> -u -b0 -P16 -t60
    NETDEV WATCHDOG: CPU: 0: transmit queue 0 timed out 2044 ms
    NETDEV WATCHDOG: CPU: 3: transmit queue 0 timed out 2004 ms

Investigation showed that the timeouts are caused by the priority-based
arbiter. Under heavy load the highest priority queue starves the lower
priority ones, causing timeouts. The TX strict priority arbiter is not
suitable for the default use case where all the traffic gets spread
across all the TX queues.

Therefore, to fix this, switch the TX DMA arbiter to Weighted Round-Robin,
which services all queues, so they do not stall. The weights were chosen
to follow the existing priority scheme: q0 gets the smallest weight, while
q1-4 get the bulk of the TX bandwidth.

Fixes: 1c1008c793fa ("net: bcmgenet: add main driver file")
Signed-off-by: Ovidiu Panait <ovidiu.panait.rb@renesas.com>
Link: https://patch.msgid.link/20260610085238.56300-1-ovidiu.panait.rb@renesas.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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