Don't relay on the doorbell register to track our slots. As of the

Don't relay on the doorbell register to track our slots.  As of the
documentation:

"UTRLDBR is a volatile register; software should only use its value to
determine commands that have completed, not to determine which commands
have previously been issued."

Instead we use the CCB structure to track our slots, as proposed by dlg@.

CAVEAT: Since using more than one slot is currently causing OCS errors,
we limit the slots to one until we can find a solution.