kernel: psb6970: drop unused variable in psb6970_config_init

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 <olek2@wp.pl>