We have now everything in place to actual let a component register controls. Add
a function which allows to do so.

Also update snd_soc_add_codec_controls() and snd_soc_platform_controls() to use
this new function internally. And while we are at it also change the
num_controls parameter of those two functions from int to unsigned int.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>