mlxsw: core: Expose FW version over defined keyword

To be aligned with the rest of the drivers, expose FW version under "fw"
keyword in devlink dev info, in addition to the existing "fw.version",
which is currently Mellanox-specific.

devlink output before:
       running:
         fw.version 30.2008.2018
after:
       running:
         fw.version 30.2008.2018
         fw 30.2008.2018

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>