ASoC: Intel: sst: Fix firmware name size handling

Function sst_acpi_probe() uses plain strcpy for setting member firmware_name
of a struct intel_sst_drv from member firmware of a struct sst_machines.
Thereby the destination array has got a length of 20 byte while the source may
hold 32 byte. Since eg. commit 64b9c90b ("ASoC: Intel: Fix BYTCR firmware
name") increased strings from "fw_sst_0f28.bin" to "intel/fw_sst_0f28.bin"
there is an actual possibility that the 20 byte array at the end of struct
intel_sst_drv is overrun.

Thus increase the size of the destination and use the same define for both
structs. Detected by Coverity CID 1260087.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org