mlxsw: Set port split ability attribute in driver

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

Set the split ability of the port as well, based on port_mapping->width
field and split attribute of devlink port in spectrum, 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>