ipq40xx: essedma: Disable TCP segmentation offload for IPv6

ipq40xx: essedma: Disable TCP segmentation offload for IPv6

It was noticed that the the whole MAC can hang when transferring data from
one ar40xx port (WAN ports) to the CPU and from the CPU back to another
ar40xx port (LAN ports). The CPU was doing only NATing in that process.

Usually, the problem first starts with a simple data corruption:

  $ wget https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-10.4.0-amd64-netinst.iso -O /dev/null
  ...
  Connecting to saimei.ftp.acc.umu.se (saimei.ftp.acc.umu.se)|2001:6b0:19::138|:443... connected.
  ...
  Read  error at byte 48807936/352321536 (Decryption has failed.). Retrying.

But after a short while, the whole MAC will stop to react. No traffic can
be transported anymore from the CPU port from/to the AR40xx PHY/switch and
the MAC has to be resetted.

The whole problem can be avoided by disabling IPv6 TSO for this ethernet
MAC driver.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: John Crispin <john@phrozen.org>
(backported from commit 678569505623e50bbbbc344c7e820fb315b79ede, with
updated commit message)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>