devlink: move linecard struct into linecard.c

Instead of exposing linecard struct, expose a simple helper to get the
linecard index, which is all is needed outside linecard.c. Move the
linecard struct to linecard.c and keep it private similar to the rest of
the devlink objects.

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