mlxsw: spectrum_buffers: Pass SBPM min_size in cells

The SBPM register configures the shared buffer allocation and
configuration per port and pool. The min_buff value is the buffer size
dedicated to this single function, and is configured in cells.
Currently, all sb_pm entries have 0 for min_buff, and therefore the
actual unit is immaterial. However, in a follow-up patch we want to add
entries with non-zero minimum.

Therefore pass the min_buff from the sb_pm table through the cell
conversion before handing it over to mlxsw_sp_sb_pm_write().

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