ASoC: Intel: sst-acpi: Fix Oops in case of missing firmware

I swear I tested missing firmware in commit e5161d79 ("ASoC: Intel:
sst-acpi: Request firmware before SST platform driver probing").

Unfortunately same wasn't done in commit 6dda27cb ("ASoC: Intel:
sst-acpi: Add support for multiple machine drivers per platform") which
will cause NULL pointer dereference in sst_acpi_fw_cb() when printing the
error since sst_acpi->mach is not set.

Fix this obvious error by setting the sst_acpi->mach in sst_acpi_probe().

Reported-by: Mika Westerberg <mika.westerberg@linux.intel.com>
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>