V4L/DVB (7556): em28xx: fix locking on vidioc_s_fmt_cap

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: Aidan Thornton <makosoft@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>