drivers/perf: riscv: Fix counter mask iteration for RV32

For RV32, used_hw_ctrs can have more than 1 word if the firmware chooses
to interleave firmware/hardware counters indicies. Even though it's a
unlikely scenario, handle that case by iterating over all the words
instead of just using the first word.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20240420151741.962500-9-atishp@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>