s390/pci: make zpci_set_irq()/zpci_clear_irq() static

Commit c1e18c17 ("s390/pci: add zpci_set_irq()/zpci_clear_irq()")
made zpci_set_irq()/zpci_clear_irq() non-static in preparation for using
them in zpci_hot_reset_device(). The version of zpci_hot_reset_device()
that was finally merged however exploits the fact that IRQs and DMA is
implicitly disabled by clp_disable_fh() so the call to zpci_clear_irq()
was never added. There are no other calls outside pci_irq.c so lets make
both functions static.

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