net: dsa: qca8k: fix wrong length value for mgmt eth packet

net: dsa: qca8k: fix wrong length value for mgmt eth packet

The assumption that Documentation was right about how this value work was
wrong. It was discovered that the length value of the mgmt header is in
step of word size.

As an example to process 4 byte of data the correct length to set is 2.
To process 8 byte 4, 12 byte 6, 16 byte 8...

Odd values will always return the next size on the ack packet.
(length of 3 (6 byte) will always return 8 bytes of data)

This means that a value of 15 (0xf) actually means reading/writing 32 bytes
of data instead of 16 bytes. This behaviour is totally absent and not
documented in the switch Documentation.

In fact from Documentation the max value that mgmt eth can process is
16 byte of data while in reality it can process 32 bytes at once.

To handle this we always round up the length after deviding it for word
size. We check if the result is odd and we round another time to align
to what the switch will provide in the ack packet.
The workaround for the length limit of 15 is still needed as the length
reg max value is 0xf(15)

Reported-by: Ronald Wahl <ronald.wahl@raritan.com>
Tested-by: Ronald Wahl <ronald.wahl@raritan.com>
Fixes: 90386223f44e ("net: dsa: qca8k: add support for larger read/write size with mgmt Ethernet")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Cc: stable@vger.kernel.org # v5.18+
Signed-off-by: David S. Miller <davem@davemloft.net>