pmu_write_counter() is now only called from pmu_write_counters(),
which does so for each set index in the given mask, bounded by
cci_pmu->num_cntrs. So, there is no need for an extra check to
make sure the given counter is valid inside pmu_write_counter.
This patch gets rid of that.

Cc: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Olof Johansson <olof@lixom.net>
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>