net: dsa: mt7530: fix corrupt frames using trgmii on 40 MHz XTAL MT7621

The multi-chip module MT7530 switch with a 40 MHz oscillator on the
MT7621AT, MT7621DAT, and MT7621ST SoCs forwards corrupt frames using
trgmii.

This is caused by the assumption that MT7621 SoCs have got 150 MHz PLL,
hence using the ncpo1 value, 0x0780.

My testing shows this value works on Unielec U7621-06, Bartel's testing
shows it won't work on Hi-Link HLK-MT7621A and Netgear WAC104. All devices
tested have got 40 MHz oscillators.

Using the value for 125 MHz PLL, 0x0640, works on all boards at hand. The
definitions for 125 MHz PLL exist on the Banana Pi BPI-R2 BSP source code
whilst 150 MHz PLL don't.

Forwarding frames using trgmii on the MCM MT7530 switch with a 25 MHz
oscillator on the said MT7621 SoCs works fine because the ncpo1 value
defined for it is for 125 MHz PLL.

Change the 150 MHz PLL comment to 125 MHz PLL, and use the 125 MHz PLL
ncpo1 values for both oscillator frequencies.

Link: https://github.com/BPI-SINOVOIP/BPI-R2-bsp/blob/81d24bbce7d99524d0771a8bdb2d6663e4eb4faa/u-boot-mt/drivers/net/rt2880_eth.c#L2195
Fixes: 7ef6f6f8 ("net: dsa: mt7530: Add MT7621 TRGMII mode support")
Tested-by: Bartel Eerdekens <bartel.eerdekens@constell8.be>
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mt7530.c

@@ -426,9 +426,9 @@ mt7530_pad_clk_setup(struct dsa_switch *ds, phy_interface_t interface)
 		else
 			ssc_delta = 0x87;
 		if (priv->id == ID_MT7621) {
-			/* PLL frequency: 150MHz: 1.2GBit */
+			/* PLL frequency: 125MHz: 1.0GBit */
 			if (xtal == HWTRAP_XTAL_40MHZ)
-				ncpo1 = 0x0780;
+				ncpo1 = 0x0640;
 			if (xtal == HWTRAP_XTAL_25MHZ)
 				ncpo1 = 0x0a00;
 		} else { /* PLL frequency: 250MHz: 2.0Gbit */