staging: dgnc: Replace kzalloc with kcalloc

This fixes the following checkpatch.pl warnings:
WARNING: Prefer kcalloc over kzalloc with multiply

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>