e1000e: Introduce private flag to disable K1

e1000e: Introduce private flag to disable K1

The K1 state reduces power consumption on ICH family network controllers
during idle periods, similarly to L1 state on PCI Express NICs. Therefore,
it is recommended and enabled by default.
However, on some systems it has been observed to have adverse side
effects, such as packet loss. It has been established through debug that
the problem may be due to firmware misconfiguration of specific systems,
interoperability with certain link partners, or marginal electrical
conditions of specific units.

These problems typically cannot be fixed in the field, and generic
workarounds to resolve the side effects on all systems, while keeping K1
enabled, were found infeasible.
Therefore, add the option for users to globally disable K1 idle state on
the adapter.

Additionally, disable K1 by default for MTL and later platforms, due to
issues reported with the current configuration.

Link: https://lore.kernel.org/intel-wired-lan/CAMqyJG3LVqfgqMcTxeaPur_Jq0oQH7GgdxRuVtRX_6TTH2mX5Q@mail.gmail.com/
Link: https://lore.kernel.org/intel-wired-lan/20250626153544.1853d106@onyx.my.domain/
Link: https://lore.kernel.org/intel-wired-lan/Z_z9EjcKtwHCQcZR@mail-itl/
Link: https://github.com/QubesOS/qubes-issues/issues/9896
Link: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2115393
Signed-off-by: Vitaly Lifshits <vitaly.lifshits@intel.com>
Reviewed-by: Timo Teräs <timo.teras@iki.fi>
Tested-by: Timo Teräs <timo.teras@iki.fi>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Reviewed-by: Dima Ruinskiy <dima.ruinskiy@intel.com>
Tested-by: Avraham Koren <Avrahamx.koren@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20251016-jk-iwl-next-2025-10-15-v2-14-ff3a390d9fc6@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>