staging: comedi: ni_at_a2150: remove 'volatile' from private data variable

As indicated by checkpatch.pl, "WARNING: Use of volatile is usually
wrong: ...", the 'count' member of the private data does not need to
be volatile.

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>