staging: comedi: ni_660x: change IOConfigReg() into a macro

The BUG_ON() in this function is unnecessary. The 'pfi_channel' will
always be in range of the subdevice 'n_chan' (NUM_PFI_CHANNELS) which
will return a valid 'reg'.

Convert the inline function into a simple macro.

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>