perf: riscv legacy: fix kerneldoc comment warning

Fix the warning:
drivers/perf/riscv_pmu_legacy.c:76: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

Fixes: 9b3e150e

 ("RISC-V: Add a simple platform driver for RISC-V legacy perf")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20220812143532.1962623-1-conor.dooley@microchip.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>