[media] media: i2c/soc_camera: fix ov6650 sensor getting wrong clock

After changes to v4l2_clk API introduced in v4.1 by commits a37462b9
'[media] V4L: remove clock name from v4l2_clk API' and 4f528afc
'[media] V4L: add CCF support to the v4l2_clk API', ov6650 sensor
stopped responding because v4l2_clk_get(), still called with
depreciated V4L2 clock name "mclk", started to return respective CCF
clock instead of the V4l2 one registered by soc_camera. Fix it by
calling v4l2_clk_get() with NULL clock name.

Created and tested on Amstrad Delta against Linux-4.7-rc3 with
omap1_camera fixes.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>