jtag: clean up TAP state name handling

Some cosmetic cleanup, and switch to a single table mapping
between state names and symbols (vs two routines which only
share that state with difficulty).

Get rid of TAP_NUM_STATES, and some related knowledge about
how TAP numbers are assigned.  Later on, this will help us
get rid of more such hardwired knowlege.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>