r8169: work around an irq coalescing related tx timeout

r8169: work around an irq coalescing related tx timeout

In [0] a user reported reproducible tx timeouts on RTL8168f except
PktCntrDisable is set and irq coalescing is enabled.
Realtek told me that they are not aware of any related hw issue on
this chip version, therefore root cause is still unknown. It's not
clear whether the issue affects one or more chip versions in general,
or whether issue is specific to reporter's system.
Due to this level of uncertainty, and due to the fact that I'm aware
of this one report only, let's apply the workaround on net-next only.
After this change setting irq coalescing via ethtool can reliably
avoid the issue on the affected system.

[0] https://bugzilla.kernel.org/show_bug.cgi?id=207205

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>