mlxsw: spectrum_buffers: Cache shared buffer configuration

In order to achieve faster dumping of current setting and also in order
to provide possibility to get pool mode without a need to query hardware,
do cache the configuration in driver.

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