target/loongarch: Fix CSR STLBPS register write emulation

Function helper_csrwr_stlbps() is emulation with CSR STLBPS register
write operation. However there is only parameter checking action, and
no register updating action. Here update value of CSR_STLBPS when
parameter passes to check.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>

target/loongarch/tcg/csr_helper.c

@@ -29,7 +29,11 @@ target_ulong helper_csrwr_stlbps(CPULoongArchState *env, target_ulong val)
     if (!check_ps(env, tlb_ps)) {
         qemu_log_mask(LOG_GUEST_ERROR,
                       "Attempted set ps %d\n", tlb_ps);
+    } else {
+        /* Only update PS field, reserved bit keeps zero */
+        env->CSR_STLBPS = FIELD_DP64(old_v, CSR_STLBPS, PS, tlb_ps);
     }
+
     return old_v;
 }