staging: comedi: ni_tio: tidy up ni_tio_get_gate_src()

Clarify the code a bit by handling the NI_GPCT_DISABLED_GATE_SELECT source
in the common code path.

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>