s390x/pci: fix stpcifc_service_call

Firstly the function misses dmaas checking. This patch adds it.

Secondly the function uses s390_pci_find_dev_by_fh() to look up the
zpci device. This may fail if the guest provides a valid and disabled
fh but fh of the associated zpci device is enabled. Thus we use
s390_pci_find_dev_by_idx() instead.

Signed-off-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
