staging: comedi: comedi_fc: cfc_write_array_to_buffer() data is const

The `data` pointer of `cfc_write_array_to_buffer()` ought to point to
`const` data.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>