staging: comedi: mite: (!foo) preferred over (foo == NULL)

Also, clarify the 'ring' allocation failure by returning NULL instead
of 'ring' (which would be NULL).

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>