net: ethtool: fix ioctl confusing drivers about desired HDS user config

The legacy ioctl path does not have support for extended attributes.
So we issue a GET to fetch the current settings from the driver,
in an attempt to keep them unchanged. HDS is a bit "special" as
the GET only returns on/off while the SET takes a "ternary" argument
(on/off/default). If the driver was in the "default" setting -
executing the ioctl path binds it to on or off, even tho the user
did not intend to change HDS config.

Factor the relevant logic out of the netlink code and reuse it.

Fixes: 87c8f849 ("bnxt_en: add support for tcp-data-split ethtool command")
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Tested-by: Daniel Xu <dxu@dxuuu.xyz>
Tested-by: Taehee Yoo <ap420073@gmail.com>
Link: https://patch.msgid.link/20250221025141.1132944-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>