target-mips: fix ASID synchronisation for MIPS MT

When syncing the task ASID with EntryHi, correctly or the value instead
of assigning it.

Backports commit 6a973e6b6584221bed89a01e755b88e58b496652 from qemu

qemu/target-mips/op_helper.c

@@ -660,7 +660,7 @@ static void sync_c0_tcstatus(CPUMIPSState *cpu, int tc,
 
     /* Sync the TASID with EntryHi.  */
     cpu->CP0_EntryHi &= ~0xff;
-    cpu->CP0_EntryHi = tasid;
+    cpu->CP0_EntryHi |= tasid;
 
     compute_hflags(cpu);
 }