[media] v4l: Casting (void *) value returned by kmalloc is useless

The semantic patch that makes this change is available
in scripts/coccinelle/api/alloc/drop_kmalloc_cast.cocci.

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>