net: macb: ignore tx_clk if MII is used

If the MII interface is used, the PHY is the clock master, thus don't
set the clock rate. On Zynq-7000, this will prevent the following
warning:
  macb e000b000.ethernet eth0: unable to generate target frequency: 25000000 Hz
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20210120194303.28268-1-michael@walle.ccSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/cadence/macb_main.c

@@ -470,6 +470,10 @@ static void macb_set_tx_clk(struct macb *bp, int speed)
 	if (!bp->tx_clk || (bp->caps & MACB_CAPS_CLK_HW_CHG))
 		return;
 
+	/* In case of MII the PHY is the clock master */
+	if (bp->phy_interface == PHY_INTERFACE_MODE_MII)
+		return;
+
 	switch (speed) {
 	case SPEED_10:
 		rate = 2500000;