cpuidle: riscv-sbi: Fix code to allow a genpd governor to be used

The intent is to use a genpd governor when there are some states that needs
to be managed. Although, the current code ends up to never assign a
governor, let's fix this.

Fixes: 6abf32f1 ("cpuidle: Add RISC-V SBI CPU idle driver")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>