ide-tape: make __idetape_discard_read_pipeline() of type void

It always returns 0 which has no effect on tape positioning calculation so
simplify it by converting its type to void, bringing no functional change to the
driver.

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