hw/display/exynos4210_fimd: Fix bit-swapping code

fimd_swap_data() includes code to reverse the bits in a
64-bit integer, but an off-by-one error meant that it would
try to shift off the top of the integer. Correct the bug
(spotted by Coverity).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1432912615-23107-1-git-send-email-peter.maydell@linaro.org
