net: dsa: microchip: lan937x: Fix RGMII delay tuning

Correct RGMII delay application logic in lan937x_set_tune_adj().

The function was missing `data16 &= ~PORT_TUNE_ADJ` before setting the
new delay value. This caused the new value to be bitwise-OR'd with the
existing PORT_TUNE_ADJ field instead of replacing it.

For example, when setting the RGMII 2 TX delay on port 4, the
intended TUNE_ADJUST value of 0 (RGMII_2_TX_DELAY_2NS) was
incorrectly OR'd with the default 0x1B (from register value 0xDA3),
leaving the delay at the wrong setting.

This patch adds the missing mask to clear the field, ensuring the
correct delay value is written. Physical measurements on the RGMII TX
lines confirm the fix, showing the delay changing from ~1ns (before
change) to ~2ns.

While testing on i.MX 8MP showed this was within the platform's timing
tolerance, it did not match the intended hardware-characterized value.

Fixes: b19ac41 ("net: dsa: microchip: apply rgmii tx and rx delay in phylink mac config")
Cc: stable@vger.kernel.org
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20251114090951.4057261-1-o.rempel@pengutronix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: olerem

drivers/net/dsa/microchip/lan937x_main.c

@@ -540,6 +540,7 @@ static void lan937x_set_tune_adj(struct ksz_device *dev, int port,
 	ksz_pread16(dev, port, reg, &data16);
 
 	/* Update tune Adjust */
+	data16 &= ~PORT_TUNE_ADJ;
 	data16 |= FIELD_PREP(PORT_TUNE_ADJ, val);
 	ksz_pwrite16(dev, port, reg, data16);