staging: comedi: ni_tio: absorb ni_tio_set_first_gate_modifiers()

This function is only called by ni_tio_set_gate_src(). For aesthetics,
absorb the code into that 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>