v4l2: use opened device caps instead of physical device ones

v4l2: use opened device caps instead of physical device ones

The same physical device can export multiple devices. In
this case, the capabilities field now contains a union of
all caps available from all exported V4L2 devices alongside
a V4L2_CAP_DEVICE_CAPS flag that should be used to decide
what capabilities to consider. In our case, we need the
ones from the exported device we are using.

https://bugzilla.gnome.org/show_bug.cgi?id=768195