net: phylink: Fix flow control for fixed-link

In phylink_parse_fixedlink() the pl->link_config.advertising bits are AND
with pl->supported, pl->supported is zeroed and only the speed/duplex
modes and MII bits are set.
So pl->link_config.advertising always loses the flow control/pause bits.

By setting Pause and Asym_Pause bits in pl->supported, the flow control
work again when devicetree "pause" is set in fixes-link node and the MAC
advertise that is supports pause.

Results with this patch.

Legend:
- DT = 'Pause' is set in the fixed-link in devicetree.
- validate() = ‘Yes’ means phylink_set(mask, Pause) is set in the
  validate().
- flow = results reported my link is Up line.

+-----+------------+-------+
| DT  | validate() | flow  |
+-----+------------+-------+
| Yes | Yes        | rx/tx |
| No  | Yes        | off   |
| Yes | No         | off   |
+-----+------------+-------+

Fixes: 9525ae8395 ("phylink: add phylink infrastructure")
Signed-off-by: René van Dorst <opensource@vdorst.com>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phylink.c

@@ -216,6 +216,8 @@ static int phylink_parse_fixedlink(struct phylink *pl,
 			       pl->supported, true);
 	linkmode_zero(pl->supported);
 	phylink_set(pl->supported, MII);
+	phylink_set(pl->supported, Pause);
+	phylink_set(pl->supported, Asym_Pause);
 	if (s) {
 		__set_bit(s->bit, pl->supported);
 	} else {