each individual ATA register is in the attachment structure, we
can dispense with the custom bus_space mapping hack which set the
stride, then unset it for particular bus_space functions. (There
is really no stride; the data register is read and written to
consecutive bytes. The control register mappings just happen to
be separated by four bytes.) While we're here, de-__P(), and lose
a confusing shift.
This has been tested on Quadra 630. Reviewed by bouyer.
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