media: v4l2-compat-ioctl32.c: add missing VIDIOC_PREPARE_BUF

commit 3ee6d040719ae09110e5cdf24d5386abe5d1b776 upstream.

The result of the VIDIOC_PREPARE_BUF ioctl was never copied back
to userspace since it was missing in the switch.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: <stable@vger.kernel.org>      # for v4.15 and up
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>