staging: comedi: ni_mio_common: (*cancel) only works with async cmd support

The comedi subdevice (*cancel) function can only be called by the core if the
subdevice supports async commands. Move the initialization of this callback
to it is only set if async commands are enabled.

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>