staging: comedi: comedi_buf: cleanup comedi_buf_read_n_available()

For aesthetic reasons, cleanup this function a bit.

Change the (async == NULL) test to simply (!async).

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

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>