s390/pci: Refactor arch_setup_msi_irqs()

Factor out adapter interrupt allocation from arch_setup_msi_irqs() in
preparation for enabling registration of multiple MSIs. Code movement
only, no change of functionality intended.

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