lib: sbi_pmu: Replace sbi_pmu_ctr_read() with sbi_pmu_ctr_fw_read()

The "read a firmware counter" SBI call should only work for firmware
counters so let us replace sbi_pmu_ctr_read() with sbi_pmu_ctr_fw_read()
which works only on firmware counters.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>