ramips: ethernet: Disable TSO support to improve stability

ramips: ethernet: Disable TSO support to improve stability

Stability of this Ethernet driver has been a long-standing issue, with
many people reporting frequent "transmit queue timeouts" and even
occasional crashes.

Disabling TSO in the driver helps with stability, although it is likely a
workaround and might not fix the issue completely.

There is a slight slowdown in forwarding performance for TCP packets
(75 kpps vs. 80 kpps with comparable CPU utilization), but this is still
enough to forward close to 1 Gbit/s of full-sized packets across multiple
flows.

Master is using a different ethernet driver, so this is not a backport.
Because of this different driver, the upcoming 21.02 release does not seem
to be affected by these stability issues.

Thanks to mrakotiq for the initial patch.

Fixes: FS#2628
Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>