staging: comedi: comedi_buf: clarify comedi_buf_write_free()

Use the helper comedi_buf_write_n_allocated() to clarify the check
to make sure the number of bytes to free is not more than the number
of bytes allocated.

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