s390/pci: rename get_zdev_by_bus() to zdev_from_bus()

Getting a zpci_dev via get_zdev_by_bus() uses the long lived reference
held in zbus->function[devfn]. This is accounted for in
pcibios_add_device() and pcibios_release_device().

Therefore there is no need to increment the reference count in
get_zdev_by_bus() as is done for get_zdev_by_fid(). Instead callers must
not access the device after pcibios_release_device() was called which is
necessary for common PCI code anyway. With this though the very similar
naming may be misleading so rename get_zdev_by_bus() to zdev_from_bus()
emphasizing that we are directly referencing the zdev via the bus.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>