V4L/DVB (10662): remove redundant memset after kzalloc

Hi there!

While having a look at the allocation of struct dvb_frontend in *_attach
functions, I found some cases calling memset after kzalloc. This is
redundant, and the attached patch removes these calls.
I also changed one case calling kmalloc and memset to kzalloc.

Regards
Matthias

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>