atm: Use ARRAY_SIZE macro when appropriate

Use ARRAY_SIZE macro already defined in kernel.h for ATM
drivers.

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
Cc: Jeff Garzik <jeff@garzik.org>
Cc: chas williams <chas@cmf.nrl.navy.mil>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>