Revert "macb: support the two tx descriptors on at91rm9200"

This reverts commit 0a4e9ce17ba77847e5a9f87eed3c0ba46e3f82eb.

The code was developed and tested on an MSC313E SoC, which seems to be
half-way between the AT91RM9200 and the AT91SAM9260 in that it supports
both the 2-descriptors mode and a Tx ring.

It turns out that after the code was merged I could notice that the
controller would sometimes lock up, and only when dealing with sustained
bidirectional transfers, in which case it would report a Tx overrun
condition right after having reported being ready, and will stop sending
even after the status is cleared (a down/up cycle fixes it though).

After adding lots of traces I couldn't spot a sequence pattern allowing
to predict that this situation would happen. The chip comes with no
documentation and other bits are often reported with no conclusive
pattern either.

It is possible that my change is wrong just like it is possible that
the controller on the chip is bogus or at least unpredictable based on
existing docs from other chips. I do not have an RM9200 at hand to test
at the moment and a few tests run on a more recent 9G20 indicate that
this code path cannot be used there to test the code on a 3rd platform.

Since the MSC313E works fine in the single-descriptor mode, and that
people using the old RM9200 very likely favor stability over performance,
better revert this patch until we can test it on the original platform
this part of the driver was written for. Note that the reverted patch
was actually tested on MSC313E.

Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Claudiu Beznea <claudiu.beznea@microchip.com>
Cc: Daniel Palmer <daniel@0x0f.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/netdev/20201206092041.GA10646@1wt.eu/
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>

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