Add condition to assignments

[ANurmi]

The range selects into rps_ruser on lines, 251, 253 in axi_to_obi.sv evaluate to [0:1] in the minimal configuration where RUserWidth is zero, resulting in a Vivado error. I assume that this should have a similar condition guard around it as on line 302 of the same file.

I don't know if the tie-off in the else branch is the general solution to this, but in this case it seems to be a sufficient fix.