No bus arbitration across transactions: serializer is dropped during RX waits, so concurrent smbus callers interleave inside one transfer

Summary

zsleep atomically releases sc->slz during RX waits and SMB_REQUEST_BUS is a stub returning 0: a second caller takes the serializer mid-A-transfer, resets rpos/rnext (discarding A's buffered data), possibly re-enables the controller, and A later consumes B's bytes as its own. Concurrent kernel users cross-deliver data (trackpad/sensor/SSIF). Real-REQUEST_BUS-exclusion fix.