ethtool: linkstate: migrate linkstate functions to support multi-PHY setups

Adapt linkstate_get_sqi() and linkstate_get_sqi_max() to take a
phy_device argument directly, enabling support for setups with
multiple PHYs. The previous assumption of a single PHY attached to
a net_device no longer holds.

Use ethnl_req_get_phydev() to identify the appropriate PHY device
for the operation. Update linkstate_prepare_data() and related
logic to accommodate this change, ensuring compatibility with
multi-PHY configurations.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>