ATM-ZeitNet: Replace one kzalloc() call by kcalloc()

* The script "checkpatch.pl" can point information out like the following.

  WARNING: Prefer kcalloc over kzalloc with multiply

  Thus fix the affected source code place.

* Replace the specification of a data type by a pointer dereference
  to make the corresponding size determination a bit safer according to
  the Linux coding style convention.

* Delete the local variable "size" which became unnecessary with
  this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>