ASoC: Intel: fix sst firmware path

All sst firmware is provided under the intel directory of the linux-firmware
tree. By default this directory structure is kept when installing on a target
system. Change the path to expect a default linux-firmware installation.

Signed-off-by: Kevin Strasser <kevin.strasser@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>