staging: comedi: ni_mio_common: convert NI_GPCT_SUBDEV to a macro

The 'counter_index' is always in range (0 to < NUM_GPCT, which is 2).

For aesthetics, convert this inline function into a macro and remove
the BUG() which can never occur.

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>