mlxsw: spectrum: Initialize advertised speeds to supported speeds

During port initialization the driver instructs the device to only
advertise speeds that can be supported by the port's current width.

Since the device now returns the supported speeds based on the port's
current width, the driver no longer needs to compute the speeds that can
be advertised.

Simplify port initialization by setting the advertised speeds to the
queried supported speeds.

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