hw/ssi/xilinx_spips.c: Avoid shifting left into sign bit
Add missing 'U' suffix to avoid shifting left into sign bit of
a signed integer.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1392988008-15938-4-git-send-email-peter.maydell@linaro.org