[media] V4L: soc-camera: call soc_camera_power_on() after adding the client to the host

soc_camera_power_on() calls client's .s_power(1) method, which can try to
access the client hardware. This, however, is typically only possible,
after calling host's .add() method, because that's where the host driver
usually turns the master clock on.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>