To disentangle the maze in msi.c, all exported device-driver MSI APIs are
now to be grouped in one file, api.c.

Make pci_alloc_irq_vectors() a real function instead of wrapper and add
proper kernel doc to it.
Signed-off-by: NAhmed S. Darwish <darwi@linutronix.de>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Acked-by: NBjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20221111122014.870888193@linutronix.de