ASoC: SOF: Intel: cnl-ipc: re-enable IPC IRQ at end of handler

Align with Skylake driver and enable the IRQ at end of handler,
instead of at beginning.

Also add an error log if we have nothing to do in this handler.

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