i82378: Cleanup implementation

- i82378 only exists on PCI bus; do not split implementation in 2 structs
- remove BARs, which are not specified in datasheet
- replace custom isa_mmio implementation by PCI bus IO region usage
- use QOM casts when required

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
[AF: Style- and QOM-related changes, dropped no_user, reverted VMSD name]
Signed-off-by: Andreas Färber <andreas.faerber@web.de>