net: thunderx: workaround BGX TX Underflow issue

net: thunderx: workaround BGX TX Underflow issue

BugLink: https://bugs.launchpad.net/bugs/1867178
[ Upstream commit 971617c3b761c876d686a2188220a33898c90e99 ]

While it is not yet understood why a TX underflow can easily occur
for SGMII interfaces resulting in a TX wedge. It has been found that
disabling/re-enabling the LMAC resolves the issue.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Robert Jones <rjones@gateworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>