atm: iphase: return -ENOMEM instead of -1 in case of failed kmalloc()

Smatch complains about returning hard coded error codes, silence this
warning.

drivers/atm/iphase.c:115 ia_enque_rtn_q() warn: returning -1 instead of -ENOMEM is sloppy

Signed-off-by: Tillmann Heidsieck <theidsieck@leenox.de>
Signed-off-by: David S. Miller <davem@davemloft.net>