mlxsw: spectrum_dcb: Convert mlxsw_sp_port_pg_prio_map() to hdroom code

The new hdroom code has certain conventions: iteration over priorities is
done through a variable named `prio', configuration is not pushed unless it
is dirty, but a `force' flag can be used to override this, updated
configuration is written to port. Convert the function
mlxsw_sp_port_pg_prio_map() to use these conventions and rename
appropriately to fit in.

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