[media] soc_camera: Use devm_kzalloc function

devm_kzalloc() has been used to simplify error handling.
While at it, the soc_camera_device_register function has been moved to
save a few lines of code and a variable.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
[g.liakhovetski@gmx.de: remove a superfluous empty line]
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>