target/arm: Stop unintentional sign extension in pmu_init

target/arm: Stop unintentional sign extension in pmu_init

This was introduced by
    commit bf8d09694ccc07487cd73d7562081fdaec3370c8
    target/arm: Don't clear supported PMU events when initializing PMCEID1
and identified by Coverity (CID 1398645).

Signed-off-by: Aaron Lindsay <aaron@os.amperecomputing.com>
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20190219144621.450-1-aaron@os.amperecomputing.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>