wishbone2lasmi: support lasmim data_width < wishbone data_width
fhdl: forbid zero-length signals
lasmicon/lasmibus: simplify dat_ack by splitting it in dat_w_ack and dat_r_ack.
dat_w must be presented when dat_w_ack is set.
dat_r is valid when dat_r_ack is set.

In the old version, write_latency and read_latency on data was managed
directly in the ports which was not easy to do.