prep: Move int-ack register from PReP to Raven PCI emulation

Register is one byte-wide (as per specification), so there is no need
to specify endianness.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
[AF: Limit access validity to size 1]
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
