s390/pci: Refactor arch_setup_msi_irqs()

[ Upstream commit 5fd11b96b43708f2f6e3964412c301c1bd20ec0f ]

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>
Stable-dep-of: ab42fcb511fd ("s390/pci: Allow allocation of more than 1 MSI interrupt")
Signed-off-by: Sasha Levin <sashal@kernel.org>