ASoC: Intel: sst-acpi: Request firmware before SST platform driver probing

We originally thought to request SST audio DSP firmware during the SST
platform driver initialization. However plain request_firmware doesn't
work in driver probe paths if userspace is not ready to handle it. For
instance when drivers are built-in.

Implementing asynchronous firmware request in SST platform driver
initialization complicates code needlessly since it anyway will fail if
firmware is missing.

This is more simple to handle by requesting firmware asynchronously in
sst_acpi_probe() and register SST platform only after firmware is loaded.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>