[ATM]: fix broken uses of NIPQUAD in net/atm

NIPQUAD expects an l-value of type __be32, _NOT_ a pointer to __be32.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>