staging: comedi: ni_tio: tidy up ni_tio_set_other_src()

Invert the counter_dev->variant test to reduce the indent level of
this function.

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>