If a spin up command can't be started becaause of a lack of openings, delay and retry the original command rather than immediately erroring out.
If a spin up command can't be started becaause of a lack of openings,
delay and retry the original command rather than immediately erroring
out.

Fixes USB drives that spin themselves down due to inactivity, as
confirmed by pirofti@ and jsg@.

ok miod@, rough agreement pending more general solution dlg@.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars