Fix PSL check for valid fifo in data during write, fixes #750

[tmeissner]

As @JimLewis reported in #750 the result of the test if data is unknown during a FIFO write is not what we want in all cases. I fixed that by using the reduction or in combination with the to_x01() function. That's also better readable code IMHO.

[JimLewis]

Any reason you did not just use:

assert always (not rst and wr -> is_x( d)@rising_edge(clkw)

[tmeissner]

The reason was that I didn't remember that is_x() exists 😉 I will update the PR.

[eine]

Thanks! Jim for reporting and Torsten for fixing! ❤️