CVE-2026-68348: ASoC: tas2781: bound firmware description string parsing

In the Linux kernel, the following vulnerability has been resolved:

ASoC: tas2781: bound firmware description string parsing

The TAS2781 firmware parser reads several variable-length description strings with strlen() before checking that the string terminator is present inside the firmware blob. A malformed firmware image without a NUL terminator can therefore make the parser walk past the end of the firmware buffer before the later size checks run.

Add a small bounded string-length helper and use it for all description fields that are parsed from the firmware buffer. Keep the existing size checks for the fixed bytes that follow each string.