perf: dsu: Use signed field for dsu_pmu->num_counters

perf: dsu: Use signed field for dsu_pmu->num_counters

We set dsu_pmu->num_counters to -1, when the DSU is allocated
but not initialised when none of the CPUs are active in the DSU.
However, we use an unsigned field for num_counters. Switch this
to a signed field.

Fixes: 7520fa99246d ("perf: ARM DynamIQ Shared Unit PMU support")
Reported-by: Dan Carpenter <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Suzuki K Poulose <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>