[media] V4L2: soc-camera: fix requesting regulators in synchronous case

With synchronous subdevice probing regulators should be requested by the
soc-camera core in soc_camera_pdrv_probe(). Subdevice drivers, supporting
asynchronous probing, call soc_camera_power_init() to request regulators.
Erroneously, the same regulator array is used in the latter case as in
the former, which leads to a failure. This patch fixes it by preventing
the second regulator request from being executed.

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