staging: comedi: ni_tio: remove unnecessary ni_gpct_variant BUG() cases

The enum ni_gpct_variant is used to handle hardware variations in the
gpct timers handled by this driver. This enum is defined as:

enum ni_gpct_variant {
	ni_gpct_variant_e_series,
	ni_gpct_variant_m_series,
	ni_gpct_variant_660x
};

For all the switch statements where all three enum values are handled,
remove the unreachable BUG(). Move the default case so it is handled
the same as ni_gpct_variant_e_series.

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>