s390/pci: Handle futile config accesses of disabled devices directly

On s390 PCI busses and slots with multiple functions may have holes
because PCI functions are passed-through by the hypervisor on a per
function basis and some functions may be in standby or reserved. This
fact is indicated by returning true from the
hypervisor_isolated_pci_functions() helper and triggers common code to
scan all possible devfn values. Via pci_scan_single_device() this in
turn causes config reads for the device and vendor IDs, even for PCI
functions which are in standby and thereofore disabled.

So far these futile config reads, as well as potentially writes, which
can never succeed were handled by the PCI load/store instructions
themselves. This works as the platform just returns an error for
a disabled and thus not usable function handle. It does cause spamming
of error logs and additional overhead though.

Instead check if the used function handle is enabled in zpci_cfg_load()
and zpci_cfg_write() and if not enable directly return -ENODEV. Also
refactor zpci_cfg_load() and zpci_cfg_store() slightly to accommodate
the new logic while meeting modern kernel style guidelines.

Cc: stable@vger.kernel.org
Fixes: a50297cf8235 ("s390/pci: separate zbus creation from scanning")
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Reviewed-by: Farhan Ali <alifm@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>