ethtool: Add forced speed to supported link modes maps

The need to map Ethtool forced speeds to Ethtool supported link modes is
common among drivers. To support this, add a common structure for forced
speed maps and a function to init them.  This is solution was originally
introduced in commit 1d4e4ecc

 ("qede: populate supported link modes
maps on module init") for qede driver.

ethtool_forced_speed_maps_init() should be called during driver init
with an array of struct ethtool_forced_speed_map to populate the mapping.

Definitions for maps themselves are left in the driver code, as the sets
of supported link modes may vary between the devices.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Pawel Chmielewski <pawel.chmielewski@intel.com>
Signed-off-by: Paul Greenwalt <paul.greenwalt@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>