Currently, vidioc_s_fmt_cap is allowed even if streaming is running on some
other fh. This is likely to cause issues.

Block use of vidioc_s_fmt_cap if someone else has claimed access to the device.
Signed-off-by: NAidan Thornton <makosoft@googlemail.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@infradead.org>