ASoC: wm_adsp: wm_coeff_{read|write}_control should use passed length

wm_coeff_{read|write}_control were using the control length rather than
the length parameter passed to them. This is not causing any issues as
the two values are currently always the same, but this needs fixed to
allow future improvements.

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