ASoC: SOF: ipc: replace fw ready bitfield with explicit bit ordering

Previously the structure used bitfields, which do not guarantee bit
ordering.

This change makes sure the order is clearly defined.  It also renames
and repurposes the field for general use.

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