[RTL] Prevent CSR write on any illegal CSR reason
[RTL] Prevent CSR write on any illegal CSR reason

Debug register access sets illegal_csr if not in debug mode but CSR
write still went ahead. This modifies the CSR write to ensure that
anything that results in an illegal CSR instruction being signalled will
prevent a CSR write.

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