ppdev: remove unused ROUND_UP macro

This macro is not used after commit 3b9ab374
("ppdev: convert to y2038 safe"), so let's remove
it.

Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>