PCI: ibmphp: Use common error handling code in unconfigure_boot_device()

Combine two error paths that emit the same message and return the same
error code.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
[bhelgaas: changelog]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>