V4L2: Fix regression, and gstreamer workaround

As discussed on http://www.raspberrypi.org/forums/viewtopic.php?f=43&t=78183, https://github.com/6by9/linux/commit/44fe4d7e3e1526e2c63347940cf0216d5c77885f introduced a regression which caused streaming >720P to fail by default. Fixed.

From http://www.raspberrypi.org/forums/viewtopic.php?f=43&t=78094, add a workaround for Gstreamer bug https://bugzilla.gnome.org/show_bug.cgi?id=726521 as it doesn't handle enum_framesizes of type V4L2_FRMSIZE_TYPE_STEPWISE sensibly.