target-arm: A64: fix unallocated test of scalar SQXTUN

The test for the U bit was incorrectly inverted in the scalar case of SQXTUN.
This doesn't affect the vector case as the U bit is used to select XTN(2).
Reported-by: NHao Liu <hao.liu@arm.com>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NClaudio Fontana <claudio.fontana@huawei.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target-arm/translate-a64.c

@@ -7708,7 +7708,7 @@ static void disas_simd_scalar_two_reg_misc(DisasContext *s, uint32_t insn)
         }
         break;
     case 0x12: /* SQXTUN */
-        if (u) {
+        if (!u) {
             unallocated_encoding(s);
             return;
         }