[NET_SCHED]: sch_atm: fix format string warning

Fix format string warning introduces by the netlink API conversion:

net/sched/sch_atm.c:250: warning: format '%lu' expects type 'long unsigned int', but argument 3 has type 'int'.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>