ide-tape: factor out DSC handling from idetape_pc_intr()

Factor out DSC handling from idetape_pc_intr() to ide_tape_handle_dsc()
helper as a preparation for adding generic ide_pc_intr() helper.

There should be no functional changes caused by this patch.

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