mlxsw: spectrum_buffers: Query shared buffer size from firmware

Instead of hard coding the size of the shared buffer in the driver,
query it from the firmware, as it may change in future devices.

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>