In preparation for providing a special allocation function for wired
interrupts which are connected to a wire to MSI bridge, split the inner
workings of msi_domain_alloc_irq_at() out into a helper function so the
code can be shared.

No functional change.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240127161753.114685-9-apatel@ventanamicro.com