Fix for handling of device driver buffer creation call
PRM 2-612 says the device driver can modify R3-R6 during buffer creation in order to change the buffer creation parameters used with BufferManager. However, the subsequent logic in DeviceFS meant that if the buffer already existed the flags (R3) never got used. In practice this only affects the magic buffer numbers 0-9 retained for BBC Micro compatibility (Keyboard/Serial in/Serial out/Printer/Sound 0-3/Speech/Mouse), and of those only Serial & Parallel are in DeviceFS, and of those Parallel had a workaround, so really only the SerialDeviceDriver gains a fix here. Tested in an IOMD ROM. Version 0.72. Tagged as 'DeviceFS-0_72'
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