net: pcs: xpcs: fix DW_VR_MII_DIG_CTRL1_2G5_EN bit being set for 1G SGMII w/o inband

On a port with SGMII fixed-link at SPEED_1000, DW_VR_MII_DIG_CTRL1 gets
set to 0x2404. This is incorrect, because bit 2 (DW_VR_MII_DIG_CTRL1_2G5_EN)
is set.

It comes from the previous write to DW_VR_MII_AN_CTRL, because the "val"
variable is reused and is dirty. Actually, its value is 0x4, aka
FIELD_PREP(DW_VR_MII_PCS_MODE_MASK, DW_VR_MII_PCS_MODE_C37_SGMII).

Resolve the issue by clearing "val" to 0 when writing to a new register.
After the fix, the register value is 0x2400.

Prior to the blamed commit, when the read-modify-write was open-coded,
the code saved the content of the DW_VR_MII_DIG_CTRL1 register in the
"ret" variable.

Fixes: ce8d608 ("net: pcs: xpcs: add _modify() accessors")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20250114164721.2879380-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vladimiroltean

drivers/net/pcs/pcs-xpcs.c

@@ -684,6 +684,7 @@ static int xpcs_config_aneg_c37_sgmii(struct dw_xpcs *xpcs,
 	if (ret < 0)
 		return ret;
 
+	val = 0;
 	mask = DW_VR_MII_DIG_CTRL1_MAC_AUTO_SW;
 	if (neg_mode == PHYLINK_PCS_NEG_INBAND_ENABLED)
 		val = DW_VR_MII_DIG_CTRL1_MAC_AUTO_SW;