net: ethtool: create and export ethtool_dev_mm_supported()

Create a wrapper over __ethtool_dev_mm_supported() which also calls
ethnl_ops_begin() and ethnl_ops_complete(). It can be used by other code
layers, such as tc, to make sure that preemptible TCs are supported
(this is true if an underlying MAC Merge layer exists).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ferenc Fejes <fejes@inf.elte.hu>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>