staging: comedi: clarify/unify macros for NI macro-defined terminals

Uses a single macro to define multiple macros that represent a series of
terminals for NI devices.  This patch also redefines NI_MAX_COUNTERS as the
maximum number of counters possible on NI devices (instead of the maximum
index of the counters).  This was a little confusing and caused a bug in
commit 347e2448 ("staging: comedi: tio: implement global tio/ctr routing")
when setting/reading registers for counter terminals.

Fixes: 347e2448 ("staging: comedi: tio: implement global tio/ctr routing")
Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>