ASoC: wm_adsp: Simplify kcontrol handling

Get rid off the wm_coeff struct and the wm_coeff_add_kcontrol()
function.  We are now using the snd_soc_card_kcontrol() function to
get the kcontrol pointers.  No need to call into ALSA code to
register the kcontrols.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@linaro.org>