mlxsw: spectrum: Refactor port buffer configuration

The sizes and thresholds of the priority group (PG) buffers are
configured in cells, which represent a specific amount of bytes.

The cell size can vary in different devices, so it's better to query it
from the firmware than hard coding it.

Refactor the code dealing with this value into different functions, so
that it will be easier to make the conversion in the next patch.

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>