s390/pci: Restore IRQ unconditionally for the zPCI device

Commit c1e18c17 ("s390/pci: add zpci_set_irq()/zpci_clear_irq()"),
introduced the zpci_set_irq() and zpci_clear_irq(), to be used while
resetting a zPCI device.

Commit da995d53 ("s390/pci: implement reset_slot for hotplug
slot"), mentions zpci_clear_irq() being called in the path for
zpci_hot_reset_device().  But that is not the case anymore and these
functions are not called outside of this file. Instead
zpci_hot_reset_device() relies on zpci_disable_device() also clearing
the IRQs, but misses to reset the zdev->irqs_registered flag.

However after a CLP disable/enable reset, the device's IRQ are
unregistered, but the flag zdev->irq_registered does not get cleared. It
creates an inconsistent state and so arch_restore_msi_irqs() doesn't
correctly restore the device's IRQ. This becomes a problem when a PCI
driver tries to restore the state of the device through
pci_restore_state(). Restore IRQ unconditionally for the device and remove
the irq_registered flag as its redundant.

Fixes: c1e18c17 ("s390/pci: add zpci_set_irq()/zpci_clear_irq()")
Cc: stable@vger.kernnel.org
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>