mlxsw: spectrum: Adjust headroom buffers for 8x ports

The port's headroom buffers are used to store packets while they
traverse the device's pipeline and also to store packets that are egress
mirrored.

On Spectrum-3, ports with eight lanes use two headroom buffers between
which the configured headroom size is split.

In order to prevent packet loss, multiply the calculated headroom size
by two for 8x ports.

Fixes: da382875

 ("mlxsw: spectrum: Extend to support Spectrum-3 ASIC")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>