drivers/perf: riscv: Return error for default case

If the upper two bits has an invalid valid (0x1), the event mapping
is not reliable as it returns an uninitialized variable.

Return appropriate value for the default case.

Fixes: f0c9363d ("perf/riscv-sbi: Add platform specific firmware event handling")

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20241212-pmu_event_fixes_v2-v2-2-813e8a4f5962@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>