PCI: vmd: Remove unnecessary pci_set_drvdata()

The driver core clears the driver data to NULL after device_release or on
probe failure.  Thus, it is not needed to manually clear the device driver
data to NULL.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>