fdc: Fix inheritence for SUNW,fdtwo

fdc: Fix inheritence for SUNW,fdtwo

Since commit dd3be7420774f7dc8f37a96ca24d07f0b6f31b3b SUNW,fdtwo's
initfn (realizefn since 940194c2369e50d91d1abf6f36d43853eea5e539)
was using SYSBUS_FDC() cast. This uses type sysbus-fdc rather than
SUNW,fdtwo.

Fix this by letting SUNW,fdtwo and sysbus-fdc both inherit from an
abstract type base-sysbus-fdc.

This allows to consolidate realizefns by using instance_init functions.
Clean up variable names and variable order while at it.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Cc: Hu Tao <hutao@cn.fujitsu.com>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Andreas Färber <afaerber@suse.de>