Fix use of obsolete variables in vhdl-mode.el.

Fix use of obsolete variables in vhdl-mode.el.

* progmodes/vhdl-mode.el (vhdl-speedbar-initialize): Don't use
obsolete alias speedbar-key-map.
(vhdl-doc-variable, vhdl-doc-mode): Use called-interactively-p.
(vhdl-index-menu-init): Don't use obsolete variable font-lock-maximum-size.