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

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: Hao Liu <hao.liu@arm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>