V4L/DVB: si470x: -EINVAL overwritten in si470x_vidioc_s_tuner()

This patch to the si470x_vidioc_s_tuner function was developed in
cooperation with Roel Kluin <roel.kluin@gmail.com>. It sets the default
retval to 0 instead of -EINVAL, identical to what is done in all other
set/get functions of v4l2_ioctl_ops. This is just as cosmetic change, as
retval is directly overwritten by the si470x_disconnect_check() anyway.

Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>