arm64: perf: Clean up armv8pmu_select_counter

armv8pmu_select_counter always returns the passed idx. So
let us make that void and get rid of the pointless checks.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>