mlxsw: spectrum: Track MTU in struct mlxsw_sp_hdroom

MTU influences sizes of auto-allocated buffers. Make it a part of port
buffer configuration and have __mlxsw_sp_port_headroom_set() take it from
there, instead of as an argument.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>