Remove the separate index value from SuperIOConfig and store
the index at reg 0 which is reserved and returns 0 on read.
This simplifies the object state.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <15b2968fd300a12d06b42368d084f6f80d3c3be5.1610223397.git.balaton@eik.bme.hu>
[PMD: Split original patch in 5, this is part 1/5]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>