ide-tape: remove tape->merge_stage

Get rid of the pipeline merge stage but retain the chrdev req caching
functionality by using a merge buffer tape->merge_bh which is flushed in chunks
of several blocks at a time. Also, remove last references to pipelining, e.g.
typedef idetape_stage_s.

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