LeAudio: Fix buffer size type conversion for the SW encoder

The LC3 software codec uses the value of -1 to indicate the invalid number
of samples, while the codec interface uses 0 (and unsigned data type).
With this fix we handle -1 gracefully.

Bug: 377177980
Test: m -j
Flag: Exempt; trivial fix - no functional changes
Change-Id: Ibc970a240fbedba0e9c0cdb887a0a7495e25a946