x86: io_apic: Prepare alloc/free_irq_cfg()

Rename the grossly misnamed get_one_free_irq_cfg() to alloc_irq_cfg().
Add a (not yet used) irq number argument to free_irq_cfg()

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@elte.hu>