PCI: dra7xx: Set drvdata at end of probe function

Set the drvdata pointer at the end of probe function for consistency with
other drivers.  We don't need the drvdata until after the probe completes,
and we don't need it at all if the probe fails.  No functional change
intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>