V4L/DVB (11293): uvcvideo: Add zero fill for VIDIOC_ENUM_FMT

When enumerating formats with VIDIOC_ENUM_FMT the uvcvideo driver does not
fill the reserved fields of the struct v4l2_fmtdesc with zeros as required by
V4L2 API revision 0.24 [1]. Add the missing initializations.

The patch was tested with v4l-test 0.10 [2] with CNF7129 webcam found on EeePC
901.

References:
[1] V4L2 API specification, revision 0.24
    http://v4l2spec.bytesex.org/spec/r8367.htm

[2] v4l-test: Test environment for Video For Linux Two API
    http://v4l-test.sourceforge.net/

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>