Smstateen: Ignore writes to read-only hstateen*[n] bits when mstateen*[n]=0

The specification states that writes to read-only bits in a RW CSR are
ignored. The hstateen*[n] bits are read-only when mstateen*[n]=0. This
PR proposes ignoring writes to read-only hstateen*[n] bits when
mstateen*[n]=0 instead of writing the bits to 0.