x86/PCI: Disambiguate SiS85C503 PIRQ router code entities

In preparation to adding support for the SiS85C497 PIRQ router add `503' 
to the names of SiS85C503 PIRQ router code entities so that they clearly 
indicate which device they refer to.

Also restructure `sis_router_probe' such that new device IDs will be 
just new switch cases.

No functional change.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2203301610000.22465@angie.orcam.me.uk