ns16550: PCI initialization adjustments

ns16550: PCI initialization adjustments

Besides single-port serial cards, also accept multi-port ones and such
providing mixed functionality (e.g. also having a parallel port).

Reading PCI_INTERRUPT_PIN before ACPI gets enabled generally produces
an incorrect IRQ (below 16, whereas after enabling ACPI it frequently
would end up at a higher one), so this is useful (almost) only when a
system already boots in ACPI mode.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>