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: 9b3e150e31 ("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>

drivers/perf/riscv_pmu_legacy.c

@@ -72,7 +72,7 @@ static void pmu_legacy_ctr_start(struct perf_event *event, u64 ival)
 	local64_set(&hwc->prev_count, initial_val);
 }
 
-/**
+/*
  * This is just a simple implementation to allow legacy implementations
  * compatible with new RISC-V PMU driver framework.
  * This driver only allows reading two counters i.e CYCLE & INSTRET.