wb_avalon_bridge: wb burst to avalon pipeline access translation

Since there is no way of knowing what kind of (avalon) bursts
are suppported by the slaves we connect to, the wishbone bursts
are translated into avalon pipelined reads rather than avalon
bursts.
Wishbone allows for bursts to be ended at arbitrary cycles
(with the 'end burst' commmand), such cases are handled by
waiting out the remaining pending reads and filtering out the
readdatavalid signal from the wb_ack signal.