staging: comedi: ni_tio_internal.h: replace NITIO_Gi_Interrupt_Enable_Reg()

The "Interrupt Enable" registers are sequential in the enum ni_gpct_register.
Replace this inline CamelCase function with a simple define.

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>