V4L/DVB (10815): bttv: Don't need to zero ioctl parameter fields

The v4l2 core code in v4l2_ioctl will zero out the structure the driver is
supposed to fill in for read-only ioctls.  For read/write ioctls, all the
fields which aren't supplied from userspace will be zeroed out.

Zeroing code is removed from enum_input and g_tuner.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>