staging: comedi: comedi_buf: don't export comedi_buf_memcpy_to()

This function is only used internally. Remove the export.

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