mlxsw: core_linecards: Probe provisioned line cards for devices and attach them

In case the line card is provisioned, go over all possible existing
devices (gearboxes) on it and attach them, so devlink core is aware of
them.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>