[media] em28xx-video: do not unregister the v4l2 dummy clock before v4l2_device_unreg...

[media] em28xx-video: do not unregister the v4l2 dummy clock before v4l2_device_unregister() has been called

Otherwiese the core refuses to unregister the clock and the following warning
appears in the system log:

"WARNING: ... at drivers/media/v4l2-core/v4l2-clk.c:231 v4l2_clk_unregister+0x8a/0x90 [videodev]()
 v4l2_clk_unregister(): Refusing to unregister ref-counted 11-0030:mclk clock!"

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>