Revert "net/mlx5e: Update and set Xon/Xoff upon MTU set"

This reverts commit ceddedc9.

Commit in question breaks the mapping of PGs to pools for some SKUs.
Specifically multi-host NICs seem to be shipped with a custom buffer
configuration which maps the lossy PG to pool 4. But the bad commit
overrides this with pool 0 which does not have sufficient buffer space
reserved. Resulting in ~40% packet loss. The commit also breaks BMC /
OOB connection completely (100% packet loss).

Revert, similarly to commit 3fbfe251 ("Revert "net/mlx5e: Update and
set Xon/Xoff upon port speed set""). The breakage is exactly the same,
the only difference is that quoted commit would break the NIC immediately
on boot, and the currently reverted commit only when MTU is changed.

Note: "good" kernels do not restore the configuration, so downgrade isn't
enough to recover machines. A NIC power cycle seems to be necessary to
return to a healthy state (or overriding the relevant registers using
a custom patch).

Fixes: ceddedc9 ("net/mlx5e: Update and set Xon/Xoff upon MTU set")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20250929181529.1848157-1-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>