ide-{floppy,tape}: remove packet command stack

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

* Add 'struct ide_atapi_pc request_sense_pc' to struct ide_*_obj
  and switch ide*_retry_pc() to use it.

* Remove needless {floppy,tape}->pc assignment from ide*_setup().

* Remove no longer needed ide*_next_pc_storage(), pc_stack[],
  rq_stack_index, IDE*_PC_STACK and DBG_PC_STACK.

There should be no functional changes caused by this patch.

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