cmd: mii: Refactor some of the MII reg dump code

cmd: mii: Refactor some of the MII reg dump code

Share the code that prints out a register field with the function that
prints out the "special" fields.

There were two arrays the register dump list, one with reg number and
name, another with a pointer to the field table and the table size.
These two arrays had have each entry match what register is referred to.
Combine them into just one table.  Now they can't not match and there is
just one table.

Add some missing consts to pointers to string literals.

The dump code was ignoring the regno field in the description table and
assuming register 0 was at index 0, etc.  Have it use the field.

Change reg > max+1 into reg >= max, which doesn't fail if max+1 could
overflow, besides just making more sense.

Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>