[ATM]: fix sparse gfp nocast warnings

Fix implicit nocast warnings in atm code:
net/atm/atm_misc.c:35:44: warning: implicit cast to nocast type
drivers/atm/fore200e.c:183:33: warning: implicit cast to nocast type

Also use kzalloc() instead of kmalloc().

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>