staging: comedi: ni_660x: remove BUG_ON(chan >= NUM_PFI_CHANNELS)

This BUG_ON can never happen. The 'chan' value comes from the comedi
core in the insn->chanspec and will always be in range for the subdevice
number of channels (s->n_chan = NUM_PFI_CHANNELS).

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>