[net v2] net: pcs: pcs-mtk-lynxi: fix bpi-r3 serdes configuration

[Frank Wunderlich <linux@fw-web.de>]

From: Frank Wunderlich <frank-w@public-files.de>

Commit 8871389da151 introduces common pcs dts properties which writes
rx=normal,tx=normal polarity to register SGMSYS_QPHY_WRAP_CTRL of switch.
This is initialized with tx-bit set and so change inverts polarity
compared to before.

It looks like mt7531 has tx polarity inverted in hardware and set tx-bit
by default to restore the normal polarity.

The MT7531 datasheet quite clearly states:
Register 000050EC QPHY_WRAP_CTRL -- QPHY wrapper control
Reset value: 0x00000501

BIT 1 RX_BIT_POLARITY -- RX bit polarity control
 1'b0: normal
 1'b1: inverted

BIT 0 TX_BIT_POLARITY -- TX bit polarity control (TX default inversed
in MT7531)
 1'b0: normal
 1'b1: inverted

Till this patch the register write was only called when mediatek,pnswap
property was set which cannot be done for switch because the fw-node param
was always NULL from switch driver in the mtk_pcs_lynxi_create call.

Do not configure switch side like it's done before.

Fixes: 8871389da151 ("net: pcs: pcs-mtk-lynxi: deprecate "mediatek,pnswap"")
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
v2:
  - send as non-RFC
  - add datasheet information from Daniel
---
 drivers/net/pcs/pcs-mtk-lynxi.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/pcs/pcs-mtk-lynxi.c b/drivers/net/pcs/pcs-mtk-lynxi.c
index c12f8087af9b..a753bd88cbc2 100644
--- a/drivers/net/pcs/pcs-mtk-lynxi.c
+++ b/drivers/net/pcs/pcs-mtk-lynxi.c
@@ -129,6 +129,9 @@ static int mtk_pcs_config_polarity(struct mtk_pcs_lynxi *mpcs,
 	unsigned int val = 0;
 	int ret;
 
+	if (!fwnode)
+		return 0;
+
 	if (fwnode_property_read_bool(fwnode, "mediatek,pnswap"))
 		default_pol = PHY_POL_INVERT;
 
-- 
2.53.0