mlxsw: spectrum: Change stats cache to be local

Change the HW stats cache to be local. Rename it for better clarity.
It holds the results of the last result of HW stats that are being read
periodically, in order to have answer for stats request immediately.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>