[media] dvbdev: replace kcalloc with kzalloc

Since the number of elements equals to 1, so just use kzalloc to
simplify the code and make it more readable.

Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>