PCI/MSI: Move pci_msix_vec_count() to api.c

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

Move pci_msix_vec_count() and make its kernel-doc comprehensive.

Signed-off-by: Ahmed S. Darwish <darwi@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20221111122015.099461602@linutronix.de