pata_platform: Fix NULL pointer dereference

pata_platform currently dereferences a NULL pointer in pata_platform_probe()
if pdev->dev.platform_data is set to NULL. This breakage was most likely
introduced by commit 5f45bc50.
Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Jeff Garzik <jeff@garzik.org>