ASoC: SOF: Intel: cnl-ipc: read all IPC registers first

Align with hardware recommended sequences, and read all IPC registers
before doing any other actions. Playing with BUSY and DONE bits may
invalidate values.

The values read may not actually be necessary but at least this
provides a snapshot of the IPC registers with no consistency issues.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>