ide-{floppy,tape}: remove request stack

* Add 'struct request request_sense_rq' to struct ide_{floppy,tape}_obj
  and switch ide*_retry_pc() to use it (there can be only one REQUEST SENSE
  request active for a given device).

* Remove no longer needed ide*_next_rq_storage(), rq_stack[]
  and rq_stack_index.

* DBG_PCRQ_STACK -> DBG_PC_STACK

* Update comments for IDE*_PC_STACK and idetape_queue_pc_{head,tail}().

There should be no functional changes caused by this patch.

Cc: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>