[media] V4L2: em28xx: tell the ov2640 driver to balance clock enabling internally

[media] V4L2: em28xx: tell the ov2640 driver to balance clock enabling internally

The em28xx driver only calls subdevices' .s_power() method to power them
down, relying on the hardware to wake up automatically, which is usually
the case with tuners. This was acceptable with the old .standby() method,
but is wrong with .s_power(). Fixing the driver would be difficult due to
a broad supported hardware base. Instead this patch makes use of the
unbalanced_power soc-camera subdevice flag to tell the ov2640 driver to
balance calls to v4l2_clk_enable() and v4l2_clk_disable() internally.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>