mlxsw: Set number of port lanes attribute in driver

Currently, port attributes like flavour, port number and whether the
port was split are set when initializing a port.

Set the number of lanes of the port as well so that it could be easily
passed to devlink in the next patch.

Signed-off-by: Danielle Ratson <danieller@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>