ASoC: wm_adsp: Correct DSP pointer for preloader control

The offset of the DSP core needs to be taken into account for the DSP
preloader control get and put. Currently the dsp->preloaded variable
will only ever be read/updated on the first DSP, whilst this doesn't
affect the operation of the control the readback will be incorrect.

Signed-off-by: Ajit Pandey <ajit.pandey@cirrus.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org