lavd/v4l2: do not fail when VIDIOC_ENUMSTD returns EINVAL without a valid match

With some (buggy) drivers, the VIDIOC_G_STD ioctl returns a std_id that cannot
be matched with any of the enumerated v4l2_standard structures (for example
std_id = 0 or std_id = 0xffffff). Do not fail when we reach the end of the
enumeration without a valid match.

Fixes ticket #2370

Note: This commit message has been modified by Giorgio Vazzana, the original
commit message was:

"Fixed regression for mandatory VIDIOC_ENUMSTD support by v4l2"
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/v4l2.c

@@ -687,6 +687,10 @@ static int v4l2_set_parameters(AVFormatContext *s1)
             standard.index = i;
             if (v4l2_ioctl(s->fd, VIDIOC_ENUMSTD, &standard) < 0) {
                 ret = AVERROR(errno);
+                if (ret == AVERROR(EINVAL)) {
+                    tpf = &streamparm.parm.capture.timeperframe;
+                    break;
+                }
                 av_log(s1, AV_LOG_ERROR, "ioctl(VIDIOC_ENUMSTD): %s\n", av_err2str(ret));
                 return ret;
             }