Make compatible with latest firmware
A documentation error and bug in BCMVideo combined with an incompatible change in the VCHIQ interface, resulting in blank video output.
- In GenCmd_GetConfigInt, in the non-error case, Z is clear (not set) if the value read is nonzero. Many of the callers (correctly) only store the result if NE was returned, i.e. Z clear.
- In the parse error case, where it constructs the return flags using an MSR, it wasn't setting Z, so the callers were storing the corrupted r0 (actually a pointer to the query string) into the workspace in such cases.
- The firmware has changed how it reports unknown values. It now responds " is unknown" rather than "=0", and this meant it was being considered as a parsing error, hence triggering the dormant bug above.
- GenCmd_GetConfigInt has now gained special-case handling of " is unknown" results, and treats them the same as "=0".
Tested on a Pi 4 with the latest firmware.