staging: comedi: ni_stc.h: remove unreachable code in RTSI_Output_Bit()

All the callers of this function check the 'channel' number with
num_configurable_rtsi_channels(). This check can never occure.
Remove it.

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>