ethtool: untangle the linkmode and ethtool headers

Commit 26c5334d ("ethtool: Add forced speed to supported link
modes maps") added a dependency between ethtool.h and linkmode.h.
The dependency in the opposite direction already exists so the
new code was inserted in an awkward place.

The reason for ethtool.h to include linkmode.h, is that
ethtool_forced_speed_maps_init() is a static inline helper.
That's not really necessary.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Paul Greenwalt <paul.greenwalt@intel.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>