net: gem: 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
error:
  zynq_gem ethernet@e000b000: failed to set tx clock rate 25000000

Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Link: https://lore.kernel.org/r/20250415150400.136723-1-martin.kaistra@linutronix.de
Signed-off-by: Michal Simek <michal.simek@amd.com>
diff --git a/drivers/net/zynq_gem.c b/drivers/net/zynq_gem.c
index 461805a..703e224 100644
--- a/drivers/net/zynq_gem.c
+++ b/drivers/net/zynq_gem.c
@@ -567,12 +567,14 @@
 	}
 #endif
 
-	ret = clk_get_rate(&priv->tx_clk);
-	if (ret != clk_rate) {
-		ret = clk_set_rate(&priv->tx_clk, clk_rate);
-		if (IS_ERR_VALUE(ret)) {
-			dev_err(dev, "failed to set tx clock rate %ld\n", clk_rate);
-			return ret;
+	if (priv->interface != PHY_INTERFACE_MODE_MII) {
+		ret = clk_get_rate(&priv->tx_clk);
+		if (ret != clk_rate) {
+			ret = clk_set_rate(&priv->tx_clk, clk_rate);
+			if (IS_ERR_VALUE(ret)) {
+				dev_err(dev, "failed to set tx clock rate %ld\n", clk_rate);
+				return ret;
+			}
 		}
 	}