mlxsw: resources: Add maximum buffer size

We need to be able to limit the size of shared buffer pools, so query
the maximum size from the device during init.

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>