target-mips: fix incorrect test for MTHLIP

The pos field in the DSPControl register is not correctly initialized.
Per documentation, the result of MTHLIP is unpredictable if the value of the
pos field before the execution is greater than 32.
Signed-off-by: NPetar Jovanovic <petarj@mips.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

tests/tcg/mips/mips32-dsp/mthlip.c

@@ -30,7 +30,7 @@ int main()
     assert(ach == resulth);
     assert(acl == resultl);
 
-    dsp = 0x3f;
+    dsp = 0x1f;
     ach = 0x05;
     acl = 0xB4CB;
     rs  = 0x00FFBBAA;