[rtl] Fix xprop issues in cs_registers
[rtl] Fix xprop issues in cs_registers

Alter how logic for csr_wreq was written to not propagate X when using
certain more pessimistic modes of x propagation in simulation.

lowRISC/ibex,lowRISC/ibex,lowRISC/ibex,lowRISC/ibex,AmbiML/ibex,AmbiML/ibex,AmbiML/ibex,AmbiML/ibex