staging: comedi: COMEDI_BUFINFO: force bytes_written to 0 if stopped

staging: comedi: COMEDI_BUFINFO: force bytes_written to 0 if stopped

The `COMEDI_BUFINFO` ioctl is used to advance the current position in
the buffer by a specified amount (which can be 0) and get the new
position.  On input, the `bytes_written` member of `struct
comedi_bufinfo` specifies the amount to advance the "write" position for
an asynchronous command in the "write" direction.  On output, the member
indicates the amount the "write" position has actually been advanced.
Advancing the "write" position is current done even if the command has
stopped and cannot use any more written data.  Change it to force the
amount successfully written to 0 in that case.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>