mlxsw: core_linecards: Probe active line cards for devices and expose FW version

In case the line card is active, go over all possible existing
devices (gearboxes) on it and expose FW version of the flashable one.

Example:

$ devlink dev info auxiliary/mlxsw_core.lc.0
auxiliary/mlxsw_core.lc.0:
  versions:
      fixed:
        hw.revision 0
      running:
        ini.version 4
        fw 19.2010.1312

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