staging: comedi: comedi_buf: clarify comedi_buf_read_free()

Reword the comment about the need for the smp_mb().

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>