staging: comedi: use unsigned sample in cfc_write_to_buffer()

Sample values in comedi are generally represented as unsigned values.
`cfc_write_to_buffer()` in "comedi_fc.h" currently uses `short` to hold
a 16-bit sample value to be written to the buffer.  Change the type of
the parameter to `unsigned short` for consistency.

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