mlxsw: spectrum_buffers: Calculate the size of the main pool

Instead of hard-coding the size of the largest pool, calculate it from the
reported guaranteed shared buffer size and sizes of other pools (currently
only the CPU port pool).

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>