staging: comedi: ni_stc.h: tidy up AO_Trigger_Select_Register and bits

Rename the CamelCase. Use the BIT() macro to define the bits.

Tidy up the ni_ao_cmd() by using a local var to mask/set the bits then
programming the register in the common code path.

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>