s390/pci: use macro for attribute creation

Introduce the zpci_attr macro to create read only sysfs attributes
to avoid duplicate code.

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>