ASoC: wm_adsp: Clean up low level control read/write functions

Physically reading and writing controls to/from the DSP are handled by
two low level functions (wm_coeff_{write|read}_control, these currently
take in a snd_kcontrol pointer but immediately pull out a wm_coeff_ctl
pointer from the private data. These functions don't handle the kcontrols
at all they just shuttle data to and from the chip and all the call
sites have a wm_coeff_ctl pointer available. This patch just passes the
wm_coeff_ctl pointer straight into these functions.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>