staging: comedi: ni_660x: remove ni_660x_get_pfi_routing()

This simple function is only called by ni_660x_dio_insn_config().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>