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

Rename the CamelCase. The bit defines are identical to NISTC_INTA_ENA_REG.
Reuse them.

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>