sapi/lsapi: fix sapi_lsapi_ub_write() return value on short write

[lazerg]

sapi_lsapi_ub_write() returns str_length - ret when LSAPI_Write() writes fewer bytes than requested. LSAPI_Write() already returns the number of bytes it wrote (LSAPI_Write_r() in lsapilib.c returns p - pBuf, or -1 on error), the same contract every other ub_write in this codebase follows, e.g. sapi_cli_ub_write() returns ptr - str. Subtracting that count from str_length gives the wrong value. Return ret instead.

Fixes #23424