net: pcs: pcs-mtk-lynxi: fix bpi-r3 serdes configuration

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>
Link: https://patch.msgid.link/20260526153239.30194-1-linux@fw-web.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>