registers are registers that overlap with others on many controllers, but
which may actually be distinct on some controllers. Right now, the two
shadows are:
- wd_status (usually overlaps wd_command)
- wd_features (usually overlaps wd_error)
Add a new helper function, wdc_init_shadow_regs(), used to initialize
the shadow register handles on controllers where they do actually overlap.
Partially from Jordan Rhody @ Wasabi Systems, Inc.
2ecdd552dc