Put WDF_ERROR in the wdc_softc, to avoid gratuitously forcing non-active drives
into single-sector mode on the next transfer. Arrange to wait for an interrupt
after wdsetctlr(); this avoids long busy-wait delays, and gets rid of the
`extra interrupt' messages (except for one immediately after autoconfig on some
machines). Replace some uses of wdsetctlr() with simply lowering wd->sc_state;
no point in forcing this immediately. Allow control operations to time out.
Enable the warnings for long busy-wait delays by default. Some other minor
things not worth mentioning.