Fix SRET clearing MPRV

Attempting to clear MPRV via sstatus is fruitless.  Use mstatus instead.

riscv/insns/sret.h

@@ -8,14 +8,14 @@ if (STATE.v) {
 }
 reg_t next_pc = p->get_state()->sepc->read();
 set_pc_and_serialize(next_pc);
-reg_t s = STATE.sstatus->read();
+reg_t s = STATE.mstatus->read();
 reg_t prev_prv = get_field(s, MSTATUS_SPP);
 if (prev_prv != PRV_M)
   s = set_field(s, MSTATUS_MPRV, 0);
 s = set_field(s, MSTATUS_SIE, get_field(s, MSTATUS_SPIE));
 s = set_field(s, MSTATUS_SPIE, 1);
 s = set_field(s, MSTATUS_SPP, PRV_U);
-STATE.sstatus->write(s);
+STATE.mstatus->write(s);
 p->set_privilege(prev_prv);
 if (!STATE.v) {
   reg_t prev_virt = get_field(prev_hstatus, HSTATUS_SPV);