x86: ioapic: Split out the nested loop in setup_IO_APIC_irqs()

Two consecutive

    for(...)
    for(...)

lines to avoid an extra indentation are just horrible to read. I had
to look more than once to figure out what the code is doing.

Split out the inner loop into a separate function.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>