mlxsw: spectrum: Query cell size from firmware

As explained in the previous patch, the cell size may change in future
devices, so query it from the firmware instead of hard coding it.

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>