staging: comedi: ni_stc.h: cleanup ni_e_series_enable_second_irq()

This function is used to enable/disable the second irq on NI e-series
boards. This irq used used to generate dma requests for the counters.

There are only 2 counters (NUM_GPCT) so the default case of the switch
can never occur.

Tidy up this function and remove the unreachable BUG().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>