mlxsw: spectrum_buffers: Reject overlarge headroom size requests

cap_max_headroom_size holds maximum headroom size supported.
Overstepping that limit might under certain conditions lead to ASIC
freeze.

Query and store the value, and add mlxsw_sp_sb_max_headroom_cells() for
obtaining the stored value. In __mlxsw_sp_port_headroom_set(), reject
requests where the total port buffer is larger than the advertised
maximum.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>