staging: comedi: ni_tio: tidy up ni_tio_arm()

Make this function a bit more consise by absorbing the GI_HW_ARM_SEL_MASK()
inline helper and combine the two switch (start_trigger) code paths.

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>