kernel: psb6970: drop unused variable in psb6970_config_init

Drop unused variable in psb6970_config_init. This variable should have
been dropped in 0806f8fc80e6 ("lantiq: add Linux 5.10 support as testing kernel").

This fix compilation warning:
drivers/net/phy/psb6970.c: In function 'psb6970_config_init':
drivers/net/phy/psb6970.c:310:28: warning: unused variable 'dev' [-Wunused-variable]
  310 |         struct net_device *dev = pdev->attached_dev;
      |
cc1: all warnings being treated as errors

Fixes: 0806f8fc80e6 ("lantiq: add Linux 5.10 support as testing kernel")
Signed-off-by: Aleksander Jan Bajkowski <[email protected]>

target/linux/generic/files/drivers/net/phy/psb6970.c

@@ -307,7 +307,6 @@ static const struct switch_dev_ops psb6970_ops = {
 static int psb6970_config_init(struct phy_device *pdev)
 {
 	struct psb6970_priv *priv;
-	struct net_device *dev = pdev->attached_dev;
 	struct switch_dev *swdev;
 	int ret;