s390/pci: use to_pci_dev

Use the to_pci_dev macro to fetch a pci_dev from a struct device
pointer.

Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>