s390/pci: inline get_zdev

Inline get_zdev to save ~200 bytes of kernel text for CONFIG_PCI=y.
Also rename the function to to_zpci to make clear that we don't do
reference counting here.

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