ethtool: add compat for devlink info

If driver did not fill the fw_version field, try to call into
the new devlink get_info op and collect the versions that way.
We assume ethtool was always reporting running versions.

v4:
 - use IS_REACHABLE() to avoid problems with DEVLINK=m (kbuildbot).
v3 (Jiri):
 - do a dump and then parse it instead of special handling;
 - concatenate all versions (well, all that fit :)).

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>