s390/pci: add zpci_set_irq()/zpci_clear_irq()

Pull the directed vs floating IRQ check into common
zpci_set_irq()/zpci_clear_irq() functions and expose them for the rest
of the zPCI subsystem. Furthermore we add a zdev flag bit to easily
check if IRQs are registered. This is needed for use in resetting a zPCI
function.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>