net: davinci_mdio: remove unnecessary dev_set_drvdata()

Unnecessary dev_set_drvdata() is removed, because the driver core
clears the driver data to NULL after device_release or on probe failure.

Signed-off-by: Libo Chen <libo.chen@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>