[NET]: ROUND_UP macro cleanup in drivers/net/ppp_generic.c

ROUND_UP macro cleanup use DIV_ROUND_UP

Signed-off-by: Milind Arun Choudhary <milindchoudhary@gmail.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>