mlxbf_gige: add "set_link_ksettings" ethtool callback

This patch extends the "ethtool_ops" data structure to
include the "set_link_ksettings" callback. This change
enables configuration of the various interface speeds
that the BlueField-3 supports (10Mbps, 100Mbps, and 1Gbps).

Signed-off-by: David Thompson <davthompson@nvidia.com>
Signed-off-by: Asmaa Mnebhi <asmaa@nvidia.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>