s390/pci: add zpci_event_hard_deconfigured()

Extract the handling of PEC 0x0304 into a function and make sure we only
attempt to disable the function if it is enabled. Also check for errors
returned by zpci_disable_device() and leave the function alone if there
are any.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>