when reading the completion queue, it helps to write where we've read up to to the completion queue head doorbell instead of the submission queue tail doorbell.
when reading the completion queue, it helps to write where we've read up
to to the completion queue head doorbell instead of the submission queue
tail doorbell.

this lets us submit more than one command to the chip.

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