V4L/DVB (10812): v4l2: Zero out read-only ioctls in one place

If an ioctl is read-only then the driver fills in all the fields.  Lots of
times drivers only care about some fields so it's best if video_ioctl2
takes care of zeroing out the entire structure before handing it to the
driver.  This saves code in each driver to do it and driver authors often
forget.

The existing memset code in some of the read-only ioctl handlers
can be deleted.

Convert a case statement to a single if statement.

Deleted a debug line from ENUMAUDOUT that was copy-and-pasted to G_AUDOUT
by mistake.

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